For those who are asking (and apparently too lazy to navigate to wikipedia):
50 Shades of Grey is a novel based on a series of "erotic" "BDSM" twilight fanfiction. [footnote]Those words are in quotes because the sex scenes are not at all sexy and apparently those actually in the BDSM community have stated she has no clue what she's talking about.[/footnote] Someone expressed an interest in publishing it to the author, so she changed all the names to avoid litigation, and voila, really poorly written bondage porn is now cheaply available.
You'd think it would end there, but somehow this train wreck has become a best seller.
Its outsold even some novels you may have heard of, like (to pick an example totally at random) Harry Potter.
While the trashy romance genre has always been fantastically successful, it only tends to be so in general rather than having a remarkably popular set of books like this, so it's quite surprising, especially considering the amusingly terrible quality of the writing.
So there you have it people who asked a random question on forums instead of using the freely available one click away resources, you have been educated.
EDIT: Okay, upon double checking it has not outsold Harry Potter, but it has beaten its record as the fastest selling paperback of all time. And has sold ten million copies worldwide.