The C dominant seventh chord is a 4-note chord consisting of the notes C, E, G and Bb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as C7.
Chords that are a superset of C7. The chords include more notes but always C, E, G and Bb.
Chords that are a subset of C7. These contain less notes but all of them are included in C7.
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 C7 chord. Feel free to save or share the image as needed.