You know you love them.
Okay, so maybe you hate them.
Maybe you skip them.
Maybe you're thinking that "prologues" are somehow "pro-logging" and you're vehemently opposed to deforestation and totally offended that I've blogged about this.... if so, I suggest you buy a dictionary and switch to de-caf.
Personally, I like prologues. I also hate them, and have been known to skip them on occasion (though I usually go back later and read it to see what happened).
I get the animosity toward them, I really do. Most prologues are terrible or gimmicky or that horrendous combination of the two that makes you want to not read the rest of the book because you're afraid the whole thing is gimmicky and terrible. But, when done right, prologues can be awesome.
The first thing a good prologue must be is NECESSARY. If you're using it as a crutch to frontload your readers with backstory, use the delete key and be merciless. Give your readers some credit and trust them to figure out the backstory they need from what you've woven into the book. If they can't get what they need from the book, then learn how to weave.
The second thing a good prologue must be is SHORT. If it's not short, you have a first chapter with identity issues. Get it some counseling, tell it to proud of who and what it is, and promptly file the necessary paperwork for a legal name change on your chapter's behalf. It may hate you know, but it will thank you later.
The third thing a good prologue must be is RIGHT. That one's a little harder to quantify in one word, but it's the closest word I can think of to what I mean. If the prologue is done the right way, then the story without it will still make sense, but the story with it will be richer for its inclusion. It doesn't steal anything from the story or cheat the readers out of the experience of discovering the characters, but highlights something that will enhance the overall experience of the tale.
Things a prologue should NOT be are:
The previously mentioned INFODUMP. You may need to know your characters' history for sixteen generations, but your readers don't. At least not at first. (I don't care that Tolkein took up half of The Return of the King with Appendices - you are NOT Tolkein. And even Tolkein didn't use them as a prologue.)
A prologue should not be BORING. If there's any chance at all that the prologue will be read by someone in a bookstore or (on-line) who is considering buying your book, you don't want their "excerpt" to put them to sleep. This will make them put the book back on the shelf.
A prologue should not be an exercise in VANITY for the author or a CHEAT on the storyline. If your action comes in so late that you need a "teaser" to assure people that there will be action eventually... sometime... somewhere... you think... then you may have a bad book on your hands - or at least one that needs to be edited.
Personally, I've used prologues for the following:
To set an omniscient narrator -- this lets the reader know that an actual being is telling the story, even if they aren't featured in it.
To set an inciting incident that happened far removed from the book's beginning -- this gives a character who never appears in the book, but impacts its plot, their (SHORT! CONCISE! TO THE POINT!) moment.
To show something that a character can see, but the reader, supposedly, can not -- think of this like a journal entry or the inscription in the book. Something like a warning... okay fine, I'll just show you what I mean.
If you are reading this book, then it hasn't been burned.
If it hasn't been burned, then your predecessor failed.
If your predecessor failed, then he was not meant to have it.
If he was not meant to have it, then you're better off not knowing what became of him.
The book is your responsibility now.
Only your hands can open it and only your eyes see what's written within. No one else can help you.
And you absolutely must understand this: There are only two ways to relieve yourself of this responsibility. Finish it -- or die trying.
This would be something that set off the whole plot, but requires a direct address to the reader him/herself.
The final "prologue" isn't a prologue at all. It's an epigraph. A short (1-3 sentences), mantra style statement that sums up the entire book's theme.
All animals are created equal. Some animals are more equal than others.
This would be the epigraph for Animal Farm. I have a story that starts with one of these kinds of statements, and I'm currently trying to decide if it should be an epigraph or a one line first
I've read all kinds of prologues. You probably have, too, but the number of them that I thought were needed is FAR below the number in existence. I hope mine don't fall into that category.