Are you supposed to down vote if you don't personally like the question, or are you supposed to down vote if the question is offensive/makes-no-sense or is just really off on any point the asker is trying to make?
From the help center
Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite: that the post contains wrong information, is poorly researched, or fails to communicate information. The more that people vote on a post, the more certain future visitors can be of the quality of information contained within that post – not to mention that upvotes are a great way to thank the author of a good post for the time and effort put into writing it!
That's basically all the guidance you're given. If it's useful, vote it up, if it's not, vote it down.