A camera is something […] is used for taking pictures.
What word should be inserted, which or what?
A camera is something […] is used for taking pictures.
What word should be inserted, which or what?
In this case, which is a standard choice. What is not, although using what in this manner has a long history in some nonstandard dialects.
As the comments say, that would also work; see the question linked by bytebuster for details, and StoneyB's answer in particular.
Both answers are actually wrong in this case.
A camera is something what is used for taking pictures.
Is just completely wrong, no question.
A camera is something which is used for taking pictures.
Needs extra punctuation to be grammatically correct:
A camera is something, which is used for taking pictures.
This reveals the true emphasis of this sentence, and shows that it's wrong. 'Which' is used to show a characteristic that applies to all the things, in this case all the "somethings," which is obviously incorrect. (There are somethings that aren't used for taking pictures!)
The correct way to say it would be
A camera is something that is used for taking pictures.