"What is that?" you ask, "Is it some sort of strange animal, perhaps? Or maybe an obscure flavor of icecream?"
Fandom (short for fanatic domain, or maybe a kingdom of fans, I've heard both) is at it's simplest, a group of fangirls (or fanboys) who obsess over a common interest.
Said interest is usually a book, movie, TV show, a band, or a celebrity.
Before you know it, you'll be a multi-fandom blog, crying over fictional characters.
Don't worry. It's not a bad thing.
Some of the popular fandoms on tumblr include the Sherlockians, Homestucks, Whovians, Potterheads, and the Supernaturalists, just to name a few.