Infodumping
In certain genres, infodumping is completely okay. Doorstopper/epic fantasies and science fiction (especially hard science fiction) rely on a certain amount of infodumping and the like to relay the details of their worlds to their readers, and readers expect that, so they don’t mind it as much.
But if you’re writing cozy anything, urban or light fantasy, romances (including historical), mysteries, light SF, or plain old fiction, you’re going to want to stay away from those. If your work is supposed to be character-driven, back stories and info dumps will absolutely hurt your writing.
An info dump is one to many paragraphs of information that you think the reader will need to have in order to make sense of your world. The character isn’t typically thinking of this information, but the narrator (you) is supplying it. I like the Honor Harrington series by David Webster, and it is chock-a-block with info dumping. History, space travel, etc. Whole paragraphs, even chapters!, of info dumping on warships. It’s all there. It’s interesting to read, the first time, but you best believe I skip those chapters when I reread because I have learned that information.
It is almost always a better idea to dribble out the information readers will need through dialogue (internal or external) or as the characters access it. For instance, if you’re trying to relate that your vampires have fangs but don’t need to sleep in a coffin, you can say, The vampire’s incisors, not her canines, had become fangs; she flashed them as she reclined on her bed and waggled her eyebrows suggestively. If you’re writing in an alternate earth setting, such as Patricia Briggs or Charlaine Harris did when writing their urban fantasy series, have the info dump woven in with the character’s daily tasks. Mercy’s coworker is a fairy, so she thinks about fairies. Sookie serves True Blood to the new vampire in town, so she thinks about vampires.
Backstories
Unless they have been recently and severely traumatized, people do not stop what they’re doing and cast their memory back to the days of yore. They just don’t. Not in any great detail. They remember the past, but it’s always in relation to what’s going on right now. When they do stop what they’re doing and remember the past in great and glowing detail, they’re having a PTSD flashback.
For instance, my boyfriend was fine, and then suddenly he was dead. He FOD’d. Dead before he hit the ground. His heart just stopped. It happened in front of me, and I remember it in exquisite detail even though it’s been five years as of this writing. For several days afterward, I couldn’t think because I kept seeing it over and over in my head. It was a very unpleasant time in my life.
When I was in second grade, I overprepared for and performed the poem “The Crocodile and the Dentist,” by Shel Silverstein, for the school talent show. I remember performing it, but not in the same kind of detail, and I certainly wouldn’t relive the moment even as I call it to memory to write about it now. And it doesn’t live rent-free in my head. Although I can still recite it perfectly, including gestures and voices, whole years go by where I don’t think of it at all, and I’m only remembering it now because I’m writing this post.
Because no one thinks about information the reader might find relevant at the time the author thinks the reader needs it, if you’re writing more than a single sentence of back story, you’re writing too much, and you’re starting to tell, not show. Back story should be threaded in, in a tantalizing fashion, as the story progresses. I’m not going to make up my own back story here, but I will excerpt a bit from Grace Burrowes’s historical romance The MacGregor’s Lady.
“Lady Balfour,” he corrected her, though he knew she was being Colonial on purpose, as he had often been Scottish on purpose, or even Mohawk.
We know he’s Scottish because his family was introduced several books ago, but the bit about Mohawk was news to me. I was intrigued. Later, we learn that he spent winters in a longhouse, that his mother was Mohawk, and that he left her tribe when he was eight. All of that information is dribbled out over 130 pages, and it’s memorable because it’s mysterious and intriguing, not because it was presented in four paragraphs on page ten, before it was valuable or necessary.
If you write out a big backstory at the beginning of the book, two things will happen. First, your readers will forget it because it is not relevant or interesting–it’s boring because we don’t care about the characters yet. Second, you have interrupted your story to tell us about your character, and, speaking as a reader and veteran TTRPG player, it is not particularly fun to listen to someone over-relate about their character. I’d rather discover the character for myself.