A E7 chord contains the notes E, B, D, G#. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with E as the root.
What is the difference between E and E7?
E7 adds a flattened seventh (D) to the E major triad. That one note turns a stable, resolved chord into a restless one that wants to move — usually to A — which is why E7 is everywhere in blues.