Pronoun

Pronoun

A pronoun is a word used to replace a noun.

James is the first choice for the post. He has applied for it twice already.
("He" is a pronoun. In this example, it replaces the proper noun "James".)
("It" is a pronoun. Here, it replaces the common noun "post".)  
Some / Who / This
(The term 'pronoun' covers lots of words, and all three words above are classified as pronouns. There is whole section dedicated to Pronouns.