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.