Pronouns are parts of the sentence which are used like a noun (Mr Baker => he, the dog => it, Toby and Molly => they).
Subject pronouns and object pronouns are called “personal pronouns” as they talk about the person of the sentence.
Possessive determiners show to which person the thing belongs (This is my classroom.). They need a noun because they show the owner of this noun (We love our house.).
Possessive pronouns haven’t got a noun, because they only show the owner of something (This classroom is mine./ My car is blue, yours is black.) It is used to keep a sentence shorter (This classroom is my classroom. => This classroom is mine.).