The D major chord is a 3-note chord consisting of the notes D, F# and A.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as DM.
Chords that are a superset of DM. The chords include more notes but always D, F# and A.
Chords that are a subset of DM. These contain less notes but all of them are included in DM.
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 DM chord. Feel free to save or share the image as needed.