Some of the Mellowest Chords Around
Minor 7th chords are some of the most-used chords there are. They are also the mellowest chords around -- entire songs have been composed using nothing but minor 7th chords, and even more songs composed with a combination of minor 7ths and maj7th chords combined.
Minor 7th chords are made of a minor 3rd with a major third over it and a minor 3rd over that. In other words, every other interval is minor, and every other interval is major.
For example, on Cm7 it is a minor 3rd from C to Eb. Then it is a major 3rd from Eb to G. Then it is a minor 3rd from G to Bb. And here's what they look like:
A minor seventh chord
consists of a minor triad plus a minor seventh above the root of the triad. A minor seventh chord appears in leadsheets as any of the following chord symbols:
Cm7 Cmi7 Cmin7 C-7
Play them over and over until you get the feel of each one. Then try playing them with your left hand while playing one or more of them broken in the right hand. That's the very beginning of improvisation!