First time I heard the term. Wikipedia doesn't know about it. I remember hearing about it through the Nanowrimo forums a year or so ago, but it was never called that. Maybe it was newer back then. But anyway, before we give you a definition, how about a sampling?
Once you are done with that, you get the idea behind the term. For more, more info, more fright, head on over to Encyclopedia Dramatica, where there is a nice definition:
Creepypasta are short stories designed to unnerve, disturb, elicit a negative emotional response from, and scare the reader.
Not to mention, the pictures really add a certain flavor to the reading experience. Anyway, there all styles of these stories, from urban legends to walk-throughs. Creepy as all hell, but inspirational too--for writers. That is why the Nanoer posted the link in the first place, to the site that started it all for me and I'll use to close with you.
So have fun, get inspired, and try not to get too creeped out in the process.