There are different Usages of the letter Q.
1. the seventeenth 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 L in a set of items, categories, sizes, etc. e.g. It also denotes the seventeenth of seventeen or more hypothetical people or things.
Q = Question when A = Answer. Besides, Q letter of the English Alphabet is used in Qatar as the International Vehicle Registration.
In Physics Q = unit of Electric Charge.
The most Common Noun which is used to learn this letter at the Nursery level is Monkey; Q for Queen.