There are different Usages of the letter U.
1. the twenty-first 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 next after T in a set of items, categories, sizes, etc. e.g. It also denotes the twenty-first of twenty one or more hypothetical people or things.
u = initial velocity as in s = ut + 1/2 at2 a Physics formula. u = You as in informal chats. Besides, U letter of the English Alphabet is used in Uruguay as the International Vehicle Registration.
In Chemistry U = Uranium the 92nd element of the periodic table.
The most Common Noun which is used to learn this letter at the Nursery level is Umbrella; U for Umbrella.