Barre chord
fret 3, A shapex35543
Cm is the C minor chord. Its notes are C, E♭ and G. The lowered third gives this triad its darker color.
x35543
8-10-10-8-8-8
Numbers show frets from the low E string to the high e string; x means don't play that string.
Cm is the ii chord in B♭ major, the iii chord in A♭ major and the vi chord in E♭ major. That makes it a natural fit in songs written in those keys.
Every C, E♭ and G on the first 12 frets. The filled dots are the root, C. Build your own shape from any nearby group.
Other C chords
Other minor chords