as a determiner (followed by a noun): Which colour do you like best?
as a pronoun (without a following noun): Choose any card – it doesn’t matter which. (introducing a question): ♦ Which is the hottest month of the year? (followed by ‘of’): ♦ Which of you is responsible for this department? (introducing a relative clause): ♦ My car, which I have owned for five years, is a Ford.