The blurb is the description of a book you see on retail websites (and sometimes on the back of the book or, if it’s hardback, the inside of the dust jacket). They’re often fairly short (200-400 words or so), and they’re also something of a bugger to write.
A blurb should aim to set out the premise of a story and make the prospective reader interested in how it gets resolved. Personally, I consider (roughly) the first three chapters to not count as far as spoilers go (if it’s in the sample zone then it’s hardly giving away the ending).
Readers need to get a feel for the story, but if you give away too much then you risk spoiling it, or just making the reader feel as though they don’t need to read it because they know how it’ll go (in the same way someone who knows how a film ends may be less inclined to watch it than someone who does not).
From a writing perspective, it varies a bit according to whether the story is about a protagonist, or whether it’s about a world with a fairly large cast (Game of Thrones being a prime example). If you tried to write a blurb for Game of Thrones focusing on a protagonist you’d either pick one and give a misleading picture (because Ned Stark, Jon Snow, Tyrion Lannister and so on could all claim that role) or end up with a mega-blurb of a dozen characters.
So, if it’s about a wide-scale/global issue (sticking with Game of Thrones, this would be the war) then write about that: who will prevail in the Seven Kingdoms? If it’s about a single character’s struggle (in Bane of Souls this would be Horst, a young man who gets effectively conscripted by mages in a foreign city just as the city is being terrorised by a serial killer who likes knocking off mages) then focus on that.
Sometimes a three paragraph approach can work well. The first paragraph has a hook (I remember Better Than Life [a Red Dwarf book] having “Rimmer has a problem. He’s dead. But that’s not the problem.”), the second fleshes out the premise, and the third leaves a dangling question, possibly with an ellipsis…
As well as the description, blurbs sometimes include quotes from authors and reviewers of the book, or possibly previous books. If you happen to get George RR Martin saying your book’s brilliant, it’s not a bad idea to let other people know this.
If the book’s part of a series it’s also a good idea to let people know in the blurb. That way they won’t accidentally buy part 2 first and struggle to get into it.
It’s a good idea to read the blurbs of other books in your genre and see how they do things. See what you like, what you don’t, and the sort of approach that would work for your book.
I find blurbs pretty tricky to get right. Perhaps this is because, when searching for books to read, I pay minimal attention to them. But some people do consider the blurb and weigh it heavily when deciding whether to try a book or not, so it’s important to try and get it right.