Intervals are simply relationships between two notes. When intervals are played together, they form certain chords, such as a major chord being formed when a note, its major third and perfect fifth are played together. The intervals are

0 half steps: Unison

1: Minor Second

2: Major Second

3: Minor Third

4: Major Third

5: Perfect Fourth

6: Augmented Fourth/ Diminished Fifth

7: Perfect Fifth

8: Minor Sixth

9: Major Sixth

10: Minor Seventh

11: Major Seventh

12: Octave

This is why chords are named certain ways. If you have a note a minor third away from the tonic, then it's a minor chord of some sort. If you have a fourth, it's a sus4.

Remember though, the names of chord only describe the relationship of the notes from the tonic/root of the chord. So if you have a G major chord of G-B-D, it's major chord even though the relationship between the B and D is a minor third.