What refers to an open set of objects or an unlimited amount of possibilities. What is it? What do you like to do? Which refers to a closed or more specific set of items, like a group. Which can refer to animals or objects but never to people, which uses 'who' instead. Which book is your favorite? Sometimes they can be used interchangeably. What/Which animals do you like? Personally, I usually use 'what' when referring to something relative or general, while I use 'which' when presented with options. 'What is your favorite color?' 'Which is your favorite color? Red, green, or blue?' This was a surprisingly tricky question. Hope this helps!