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