* who - used to ask about a person or people
* whom - used as the object of a verb or preposition, but it's becoming less common
* whose - used to ask about possession
* what - used to ask about things or ideas
* which - used to ask about a specific item from a group
Examples:
* Who is going to the party?
* Whom did you call?
* Whose book is this?
* What is your favorite color?
* Which shirt do you want to wear?