A minor seventh is the chord you get when you take a minor triad
and add to it the seventh note of the natural minor scale
. They're notated Cm7
or occasionally C-7
For example, an E minor triad is E G B. The seventh note of the E melodic minor scale is D, so the Em7 chord consists of E G B D.
Compare with the major seventh, minor major seventh, diminished seventh and dominant seventh chords.