Hello! In the last few months this subreddit has seen a number of frequently-asked questions pop up regularly. In order to hopefully alleviate some of that, I offered to write up a general FAQ post for some of these questions. This certainly isn't an exhaustive list of all the questions we see around here, nor is it incredibly detailed. However, it is a fairly comprehensive summary of many of the questions and answers we see.

ELI5 US Elections / What are primaries? / What are superdelegates?

This is a broad question, but variations of it are asked fairly regularly. Also, some more specific questions that are more common necessitate some general discussion on primaries. So this is a summary of how US elections work.

In the US, there are two main parties. (More on why there are only two is coming later in this post.) These parties compete against each other in November's general election. Before they get there, though, each party needs to select its Presidential and Vice Presidential candidates. Because the parties are sort of private entities, they're allowed to select their candidates how they want. This is what the primary elections are for.

The Democrats and Republicans have conventions in late summer in which they officially select their candidates for the general election. The people who go to these conventions and actually select the party candidates are called delegates. Each party has a whole bunch of delegates (they set the number every election, those numbers can change, and the two parties need not have the same number of delegates.) These conventions have a number of rounds (called ballots; i.e. the first ballot, the second ballot, etc.) If no candidate wins 50% 1 of the delegates on the first ballot, it goes to a second ballot, and so on. Lots of the rules go out the window when the second ballot comes, but more on that later.

Most of these delegates are bound. Bound delegates are required to vote in the convention for the candidate to whom they're bound. Primary elections are what bind the delegates. When Clinton wins a state and gets 30 delegates to Sanders' 15, for example, that means 30 delegates are bound to Clinton for the convention and must vote for her. All of the Democratic primaries are proportional, so they're split up based on the election results. Some of the Republican states are winner-take-all; if one person gets 50% of the votes, they get all of that state's delegates.

Superdelegates are not bound. They're typically party elites (state governors, members of Congress, etc.) They can declare themselves early, but are able to change their mind at any time up until they vote in the convention. (That's why they're super!) Both parties have some superdelegates, but the Democratic party has way more. (About 7% of the Republicans' delegates are super, versus about 15% for Dems). In short, superdelegates exist to give the party itself some kind of control over who represents them in the general election. They were instituted to prevent the party from having to run a weak candidate.

So basically: the parties have primary elections to determine who the candidates are. Those candidates will run against each other for November's general election.

ADDENDUM (3/27): I've seen a few questions regarding the WA/AK/HI primaries, each following some variation of "How did Bernie only get X delegates when the state had XX delegates?" Voting results take more than an hour to process. Most of what you'll see in places like CNN are estimates based on the votes that have actually been processed; the totals are constantly updated when new information comes in. Many states also assign a small number of delegates to the overall winner of a state. Those obviously won't be given out until results are certified. If a state is known to give out 101 delegates and the number of delegates given out doesn't equal 101, just wait until the results are certified.

Furthermore, Washington State's primary is multi-staged. "Western Saturday" was only the first round of delegate selection. Its delegate total won't reach 101 until its primaries are done.

ADDENDUM (4/13): Delegate math is weird in caucus states. It's not a simple "Percentage of the Vote * Number of Total Delegates = Number of Bound Delegates."

Take Wyoming, for example. (I'm borrowing from /u/Wrath-of-God's excellent comment on this.) It has a couple different kinds of delegates: 8 Congressional District delegates, 4 at-large delegates, and 2 party leader/elected official delegates. Because Democratic caucuses/primaries are proportional, these delegates are split. But they're split by class, not as a lump sum. So the 8 delegates are divided according to the vote totals. Bernie got 55.7% of the vote, which is enough to get four delegates. It's just shy of the threshold needed to take five. So Hillary gets the other four. 55.7% gives 2 at-large delegates and one PLEO delegate. So yes, Bernie won by eleven percentage points, but they tied in bound delegates. These are rules set up by the state's party and both candidates knew about them well ahead of time.

What happens to a candidate's bound delegates if the candidate drops out before the convention?

This question comes up every time a candidate drops out of the race. The short answer is that it depends on the state's rules for bound delegates. I'll offer a brief overview of the things that can happen, using Rubio as a specific example since he's the most recent. (See here for more detailed discussion of these points.)

1. Some delegates are immediately unbound when the candidate exits the race. Rubio's delegates from states like Wyoming and New Hampshire are unbound. The remaining candidates (Trump, Cruz, Kasich) can court them however they want.

2. Some delegates are bound for the first one or two ballots at the convention. These delegates will vote for Rubio on the first ballot, but are then unreleased for future ballots. Rubio can't win the nomination on the first ballot, so these delegates will only become unbound if neither Trump nor Cruz wins the majority of the delegates on the first ballot.

3. The rest of the delegates become unbound whenever Rubio releases them. He can release them to certain candidates or release them to do what they want.

The rules vary from state to state, but that's the gist of it.

A small aside based on some points from the last two questions: If no candidate wins the majority of the votes in the first ballot at the convention, all of the delegates become unbound. Every single delegate is then free to vote for whomever they want at the convention.

How does a person win a primary with only 1% of the votes in? What if the other 99% vote the other way?

This is a question that pops up after every primary. Whenever primaries happen, you'll see what are called exit polls. Exit polls are done at many polling locations and offer data on who people voted for and what those voters are like (demographics like age, race, etc.) States are 'called' with only 1% of the votes in if exit polls indicate a clear winner. (n.b. These projections aren't based on the 1% of voting receipts but on the exit polls.) This necessitates that the exit polls are truly randomly sampled, which can pose some problems. For example, some people may not want to respond to exit polls, and there could be some unobserved difference between responders and non-responders. However, there are ways to overcome biased samples, so it's not as if there are huge fundamental problems with calling elections like this. States aren't typically called early if exit polls indicate a close race. If the margin is 2-to-1, however, it's easy to call a state ahead of time.

Why are there only two parties? / How viable is a third party candidate?

There's no law or line in the Constitution that necessitates two parties. (In fact, parties in general weren't really anticipated by the Founders.) But political parties exist, and there are only two parties (as opposed to three or more) for a reason. I'll first talk about why parties exist, then talk about why there are only two.

I'd be remiss if I started an explanation on why parties exist without pointing to John Aldrich's "Why Parties?" It's a fantastic book and goes into great detail about the history and theory behind political parties. In short, political parties are a tool to make politics easier. Politicians are ambitious people. Mayhew (1974) refers to politicians as "single-minded seekers of reelection." Putting aside a discussion on whether or not this is a good thing, political parties help political actors seek and obtain reelection. Political parties help solve the collective action problem by organizing voters and aggregating their interests. Political parties also serve as a heuristic for voters, helping them make voting decisions without needing to spend much time researching. Basically, political parties were created by political actors to make winning elections easier.

So given that political parties exist, why does the US only have two? This has to do with the way US elections work. Our elections are first past the post, single-member district elections. This means the first person to get 50% 1 of the votes wins the entire seat/district. Such electoral systems tend to have two parties, a tendency first discovered by Duverger. Voters have a single vote which can be cast for a single person in an election for only a single seat. This leads to two processes by which small parties don't enter the process: fusion and elimination. Sometimes, small parties will fuse together to broaden their appeal and gain a larger number of votes. Other weak parties are eliminated because they fail to win elections and voters end up abandoning them.

This isn't to say that parties never change. Indeed, both the Republican and Democratic parties evolved from previous parties like the Whigs and the Federalists. However, in countries like the US, a party will only come to prominence at the expense of another party. (Republicans essentially replaced the Whigs, for example.)

There's a deeper discussion we could have involving game theory and spatial models, but this should suffice for a general overview.

ADDENDUM (4/13): Can Candidate X run as a third party candidate if they don't win the nomination? The quick answer is "Yes, they can..." The proper is answer is "... but they'll be far behind, almost certainly won't win, will only serve as a spoiler, and most have said they wouldn't do that." States have various deadlines for filing paperwork, and many of those deadlines have already passed. Michigan, for example, also has a sore-loser law that applies to presidential elections. Sore-loser laws apply to candidates who lost primaries and prevents them from filing as independents for the general election. For a variety of reasons like this, a Sanders or Trump Independent run would start at a significant disadvantage because they simply wouldn't have access to well over a hundred electors. So yes, they could run as Independents if they wanted to, but for myriad reasons they almost certainly couldn't win.

Why does Bernie do so poorly with minority/African-American voters?

This question is fairly common, especially when news articles break down primary election results by demographics. In short, Bernie's not doing very well among minority voters. Hillary is carrying the south largely because of the overwhelming support she gets from minority voters. This comment is actually a fantastic write-up about why Bernie is performing relatively poorly among minority voters. It's written to focus specifically on black voters, but much of the intuition applies to other minority groups as well. Anyway, here's the tl;dr of the three main points:

1. Black liberalism and white liberalism are two different things. For example, black liberals are much more religious than white liberals. For black liberals, religion is a fundamental part of their identity, and this simply isn't prevalent among white liberals. Blacks see church as liberating, but whites often see it as oppressive (see: LGBT rights.) Culture war social issues also aren't as important to blacks as they are to whites.
2. Black voters feel economic improvements that white voters do not. They also feel more optimistic about the economy than white voters do. While markers like unemployment still remain high among minorities, blacks remain optimistic about the economy. (See here for more discussion on this point.) The economy is just starting to work for them, and Bernie wants to overthrow it. Furthermore, blacks also feel political insecurity that whites don't. Charleston, Tamir Rice, Sandra Bland. While Bernie certainly has responses to these issues, he doesn't offer the same kind of security that Clinton does. To black voters, electability is a strong signal. It also helps that Clinton has tied herself closely to Obama's legacy and Obama remains the most popular black politician in decades. Her message of inclusion, rather than Bernie's message of revolution, is particularly appealing.
3. Finally, Bernie's minority outreach simply hasn't been strong. He's certainly picked up some strong endorsements from prominent black people, but he just doesn't have strong minority surrogates that he needs to overcome his shortcomings among black voters. Yes, Bernie got arrested ~50 years ago protesting. But he didn't spend nearly two decades visiting, networking, fundraising, pushing legislation, and otherwise helping black communities become a part of the political establishment the way Clinton has. (See more discussion on this point here.)

That's all I've got so far. If there are other commonly-asked questions that pop up, I'll likely update this post with a comprehensive answer. Feel free to let me know if I've missed something or need to clarify something. I hope this serves as a nice primer on US elections and answers many of the questions that are commonly asked around here. Thanks for reading!