What or Which?

We use which to form questions when we're thinking about a small number of possibilities, perhaps 2 or 3. For example:
We can go this way or that way. Which way shall we go?
There are three umbrellas here. Which is yours?
Which color do you prefer, green or blue?

We use what more generally, when there could be many possiblities:
What sort of music do you like?
What color are his eyes?
What day is it today?

Here's a short video called Hey You, What Song Are You Listening To?