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.