E is built from E, G# and B. E is the root, G# the major third and B the fifth. Said aloud, its notes are E, G sharp and B.
From the low string to the high one you hear E, B, E, G#, B and E. E rings three times across the six strings, and it is the root. The lowest and highest notes are both E, two octaves apart, on strings 6 and 1.
E lets three strings ring open. String 6 gives E, string 2 gives B and string 1 gives E. Everything fits between frets 1 and 2. Besides the main one there are two more shapes, at frets 7 and 12.
Em is one change away. String 3 is left open, bringing in G, the minor third of Em. It shares two notes with C#m, E and G#. The change that keeps the most fingers still is to E7. Two stay in place.
You will find E in seven different keys. In E major it is the I, in A major the V and in B major the IV. The rest appear in the key list. Further down there is a song that uses it.