Flap copy, maybe? If so, it really only has to set the stage: who's the victim, who's the hero, what's he/she up against. It should do so in dramatic terms, though. It's actually really hard to write well, for me, because it's so reductive and so blatantly promotional. My editor and/or her staff have done the lion's share of mine so far, although I've tweaked it here and there, given the opportunity.
It's advertising. Make it a tease. Give just enough info about the premise and the stakes for the protagonist so the reader will open the book. Most buyers look at the cover, read the cover copy, and then read the first page or a random page near the middle. You'll probably use that cover copy a lot - in media releases, web site and blog, in social space postings, brochures - lots of places in the marketing campaign.
Yes, I've looked at some back covers and came up w 290 words for it.
But the question remains: does it have to be written in the same order as the story i.e. if there's a murder at the end of chapter one, do I start with the murder and go on to describe the MC, setting & circumstances, or do I start w the descriptinof the MC, setting a.s.o.?