It can act as different parts of speech depending on its function in a sentence, but its core identity is as a pronoun.
Here are some examples:
Pronoun (subject):
* It is raining outside.
* It was a beautiful day.
Pronoun (object):
* I saw it on the table.
* I like it very much.
Pronoun (predicate nominative):
* The problem is it.
Pronoun (object of a preposition):
* I put the book on it.
Pronoun (expletive):
* It seems that you are right.
Pronoun (dummy subject):
* It is getting late.
Let me know if you'd like more examples or explanations!