Whereas a melody or chord progression written in a major key uses notes from a major scale, a melody or chord progression written in a minor key uses notes from a minor scale. Songs in a minor key usually end on the I minor chord. E.g., songs in the key of A minor usually end on an Am chord.
The most important chords in a minor key are the Im, IVm and V7 chords. In the key of A minor, these chords are Am, Dm and E7.