Yep, they're usually a teaser of the story, written in such a way as to convey the themes and tone of the novel. Simple, then, eh!?!
I don't think there are any hard-and-fast rules.
If you're asking the question as an aspiring writer who wants to be published, I think that generally marketing/publicity departments of the publishing house write the blurb, not the author (but I could be wrong!).