A flashback is “an interruption of the chronological sequence (as of a film or a literary work) of an event of earlier occurrence.”
There are so many blog posts and articles out there on flashbacks. People have different views on them. Here, on a blog with posts written by agents, is an interesting post on the effective use of a flashback. Some of the advice is write it as a complete scene, don't put it in the beginning of the book, insert a flashback after a powerful scene in the novel, give the time and place in the first sentence so as to not force your reader to try and guess it.
Then there's a different take on flashbacks. Not that they're necessarily wrong, but there is a different way to structure them and use them. Kristen Lamb – an author who if you're not already following her blog I'd start following it as soon as possible – has her own viewpoint on flashbacks. You can find her posts here, here, and here.
There are several questions you need to ask yourself before inserting a flashback. You can find a good list of them here, but let me share just a couple with you: do you need to explain the character this much; will your story work without the flashback; is there any other way you can fill your audience in on the info.
I've run into a little of a quandary with Marked. Other than it's getting a total rewrite because it turned into goo in the middle and ended up making a mess of itself, I start the book with a flashback. It's in the form of a nightmare, a dream that has come to herald something bad happening to Aisling and those around her. Now, do I cut that and do it another way? I have her explain it later in the story when she's asked. Do I leave it in? What do I do with it? Right now...I don't know. I'm going to have to sort that out when I get back to it.
What are your views on flashbacks? Do you get jarred out of books or films when they're used? Have you used them yourself?