There are different Usages of the letter J.
1. the tenth 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 I in a set of items, categories, sizes, etc. e.g. It also denotes the tenth of ten or more hypothetical people or things.
2. J as Jockey
RJ = Radio Jockey, VJ = Video Jockey, DJ = Disc Jockey and IJ = Internet Jockey.
J = Jack as in the deck of Playing Cards. Besides, J letter of English Alphabet is used in Japan as International Vehicle Registration.
In Physics J = Joule named after James Prescott Joule which is the SI unit of work or Energy that equals to the work done by a force of one Newton when its point of application moves one meter in the direction of action of the applied force and also equals to the one 3600th of one watt-hour.
5. Imaginary Number
j = imaginary number in electronics and electrical engineering, which equals to the square root of minus one.
The most Common Noun which is used to learn this letter at the Nursery level is Jug; J for Jug.