[Vocabulary] What or which?

Sometimes is hard to make a decision between what and which. These two words have the same meaning, but are used in different situations. Everybody knows that "What is your name" is a right choice. So, what's the rule? The most general says:

What:You must say What in the beginning of a question asking for information specifying something.
Which: asking for information specifying one or more people or things from a definite set.

Examples:

What color is it?
What is that behind the church?
What day is today?
What do you have in your pocket?
What will you do tomorrow ?
What did we did?


Which of the musics in my new cd do you prefer?
Which of the following are the one?
Which of your classmates got pregnant?
Which is better, go to the movies or watch the show?
Which is your favorite, black or white?
Which of the suspects murdered him?

Besides, you'll also have an opportunity to bring into play which in sentences like: Gravity is such an incredible thing, which can hurts you.
In this case, which was used referring to something previously mentioned when introducing a clause giving further information.
Examples:

Is too bad to have a brother, which don't let you have your own time.
He spent the night in the house of the evil spirit which was no longer able to live there.
Tell me which days you're free.

Com certeza, uma boa introdução ao assunto.