There are different Usages of the letter E.
1. the fifth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the fifth in a set of items, categories, sizes, etc. It also denotes the fifth highest class of academic mark. It also depicts the lowest socio-economic category for marketing purposes. E = second Vowel.
2. Music & Chess
In Music it denotes the third note of the diatonic scale of C major. And, in Chess, it denotes the fifth file from the left, as viewed from White's side of the board.
It is used to denote East Direction. E = Electronic as in E-Cash or E-Mail. Besides, E letter of English Alphabet is used in Spain as International Vehicle Registration.
In Science, E = Electric Field Strength; E = Electromotive Force; E = Energy.
three E's = Economy, Efficiency and Effectiveness.
The most Common Noun which is used to learn this letter at the Nursery level is Elephant; E for Elephant.