What are Pseudonyms?
Pseudonyms are very simple things. Often confused with other things in the language like homonyms, synonyms and antonyms, due to the presence of 'nym'.
Pseudonyms are merely any sort of name which is not the real name of a person or a thing but is very popular just like nicknames of a person.
Why are they used
They are used for many purposes. They are used so as not to let the real name of someone be in the limelight and between the public. The real name is to be kept secret and the pseudonym shall be popularized. Moreover, the pseudonym will help someone assist with an alternative name, easy to be pronounced and which can be easily recognized by other people too.
You may consider any type of nicknames pseudonyms.
For example - Mahatma Gandhi is a pseudonym of Mohandas Karamchand Gandhi. However, Baapu is also a pseudonym for Mohandas Karamchand Gandhi, so this, in fact, clarifies that there can be hundreds of pseudonyms for any name.