Beware the Info Dump
Back-story: it's the bane of every writer.
If you aren't familiar with the term, back-story refers to the events that precede the opening of a book. See, as much as we want to pretend that stories all have a beginning, the truth is, there is a whole series of events that happened before the book actually starts.
Take the Harry Potter novels for example. Even before we read about Hagrid dropping off baby Harry at his aunt and uncle's house in the first book, a whole lot of things have already taken place. Tom Riddle grew up and became Lord Voldemort who terrorized the wizarding world in turn making Harry's parents join the revolution… You get the picture. Every book has a thousand other stories behind it.
So the problem isn't the back-story per se. No, it's how to put all that back-story into novel without boring your readers to death.
Some beginning writers make the mistake of using what's called an info dump. That is, they spend pages and pages giving the entire lineage of the main character (and sometimes even all of his friends and neighbors) and the past five-hundred years of history of the magical kingdom the main character lives in, and the kinds of food everyone likes and so forth.
Yes, that's right – it's dull.
Right now, I am reading a rather fascinating sci-fi novel in which the author used conversation between characters as a way to relate back-story information to the reader. Now while this is much better than an info dump, an extremely lengthy conversation can rapidly grow boring as well. After a page or two, my eyes start to glaze over, and I can't remember which character was speaking.
When I am tempted to write too much back-story or depend too heavily on conversation, I try to do the following:
Keep my info dumps confined to a single paragraph, and then spread those paragraphs throughout the story so that they don't clump together.
Limit my conversations to a page. (That would be about 250 words.)
Find inventive ways to inform the reader. For example, if I was writing about a new type of game, I know that I don't need to list every rule. Instead, I'll try something like, "Ted's skill at finding victims in Carnage earned him the highest score in his school." It's okay to leave some things to your reader's imagination.
But whatever you do, just remember: stay away from the info dump!