I think in terms of a scale's relationship to chords. Every chord is build (or "spelled") from notes in the major scale. They are often designated with roman numerals naming the note numbr in the scale for which the chord is named.
In the key of G, we have
G major: I, G-B-D which are the 1st, 3rd, 5th notes of the G major scale. Throw in the F# (7th) and we have a G maj 7
C major: IV, C-E-G, 4th, 6th, 1st. Throw in a B (3rd) and we have a Cmaj7
D major: V, D-F#-A, 5th, 7th, 2nd. Throw in a C and we have D7. This is the dominant 7 and is muy importante in jazz and pop.
The chords built off the II, III and VI are minor chords. The VII chord is a "half diminished" or "minor 7 flat 5".
If you play the G major scale from E to E (instead of G to G), you have E minor.
It goes on and on, but this should give you something to chew on.