C#m is C#, E and G# — C# with the third lowered a semitone.
It sounds darker than a major triad, and just as settled. The i of a minor key, and the vi, ii or iii of a major one.
Tonic — minor chords rest as readily as major ones.

| C# | E | G# |
|---|---|---|
| 1 | ♭3 | 5 |
Also written C#min, C#- or C# minor. The same chord either way; which spelling you meet depends on who wrote the chart.
C#m in 1st inversion puts E in the bass instead of C#, the 3rd of the chord. The notes are the same; what changes is what the bass line does, which is most of why you would reach for it — it lets the lowest voice move by step where the root would have jumped.
2 of C#m’s 8 common shapes do this:x7665xx-x-14-13-14-12
All 8 common shapes as fret numbers, lowest string first. Hover or tap one to see it drawn.
How you get to it. The turn that lands on C#m is D#m7♭5 to G#7♭9: the ii and the V of C# minor, which is where a minor tonic is usually arrived at rather than simply begun on.
In which keys. Every note of C#m belongs to C# minor (the i), E major (the vi), F# minor (the v), Ab minor (the iv), A major (the iii) and B major (the ii).
Every note of C#m is in C# minor pentatonic, C# blues, C# dorian, C# natural minor and C# phrygian.
Change one note of C#m and you have C# (the minor third becomes the major third), C#dim (the fifth becomes the flat fifth), C#sus2 (the minor third becomes the second), C#sus4 (the minor third becomes the fourth), C#m6 (add the sixth) or C#m7 (add the flat seventh).
C#m is a minor chord — root, minor third, fifth. Minor chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.