Who: Refers to people.
* Who is coming to the party?
* Who wrote this book?
Whom: Also refers to people, but is used as the object of a verb or preposition.
* Whom did you call?
* To whom did you give the gift?
Whose: Refers to possession.
* Whose car is this?
* Whose ideas were the most interesting?
What: Refers to things or ideas.
* What is your favorite color?
* What did you learn in school today?
Which: Refers to a specific thing or person from a limited group.
* Which one do you want? (out of a choice of two or more)
* Which day will you be available? (out of a specific set of days)
Why: Asks for a reason.
* Why are you late?
* Why did you choose that option?
When: Asks for a time.
* When will the meeting be held?
* When did you last see him?
Where: Asks for a location.
* Where did you find this?
* Where do you live?
How: Asks for a manner or method.
* How did you do that?
* How are you feeling?
Note: In spoken English, "who" is often used in place of "whom," even in cases where "whom" is technically correct.