C minor chord. The Cm consists of the tones C (1), Eb (3) and G (5). In the chord shape on your guitar these tones can appear in any order and some of them may also appear more than once. The root is normally played as the lowest tone.
Like the major chord, the minor chord can be assembled by stacking two 3rd over a root. But in minor the order is contrary. For our Cm chord that means you go up a minor(!) 3rd first, getting Eb and then stack a major(!) 3rd above the Eb, ending up with G as perfect 5th.
On one string on the guitar:
minor 3rd = 1 whole-step + 1 half-step = 3 frets up
major 3rd = 2 whole-steps = 4 frets up
Let's compare major and minor chords again one below the other:
major 3rd (3) + minor 3rd (m3) = perfect 5th (5) Cm chord = C (1), E (m3) and G (5)
minor 3rd (m3) + major 3rd (3) = perfect 5th (5) Cm chord = C (1), Eb (m3) and G (5)
You can notate the C minor chord as Cm or C-, but readability may suffer when using a minus instead of the m and writing C- directly on a staff. You may also find minor chords written with just the root as lowercase letter in older song books, but I would recommend to avoid that outdated and obsolete way of writing.
When you stack 3rds on each note of a scale, you get the so called diatonic chords of that scale. Carried out that way, you will get minor chords on the II. (2nd), III. (3rd) and IV. (6th) scale degre of a major(!) scale. The 6th scale degree is the so called key of parallel minor, but usually scale degrees are measured based on the major scale (except e.g. II-V-I or IV-V-I progressions in minor, where the key of parallel minor is considered as the 1st scale degree.
So in a song in the key of C major (respectively A minor) you may most likely find the chords Dm (IIm), Em (IIIm) and Am (VIm). Consequently we can find our Cm chord most likely in a song in the key of Bb major (Cm = IIm) = G minor (Cm = IVm), Ab major (Cm = IIIm) = F minor (Cm = Vm) oder Eb major (Cm = VIm) = C minor (Cm = Im).
The chord of each type relate to eachother in the same way:
Major scale: major chords on Ist, IVth and Vth scale degree
Minor scale: minor chords on Ist, IVth and Vth scale degree (equates IInd, IIIrd and VIth scale degree of the major scale).
...the VIIth scale degree (major scale) is still missing: here we can find a diminished chord (dim)...