Here are all the chords in the key of C.

As you can see chords I, IV and V are major chords, and chords II, III and VI are minor. Chord VII is a different kind of chord called Diminished. Diminished chords consist of a minor 3rd and a diminisished 5th interval above the root. Check out the section on Intervals in The Basics if you’re not sure what a diminished 5th is.

This order of major, minor and diminished chords is the same for every key. So if you take the key of G and build a chord triad on each note from the G major scale, you’ll get the same pattern of chords: I major, II minor, III minor, IV major, V major, VI minor, VII diminished.