The B major chord is a 3-note chord consisting of the notes B, D# and F#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as BM.
Chords that are a superset of BM. The chords include more notes but always B, D# and F#.
Chords that are a subset of BM. These contain less notes but all of them are included in BM.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the BM chord. Feel free to save or share the image as needed.