Creepypasta as defined by Oh Internet:
Creepypasta is generally a story posted on a forum, image board, or sent through e-mail telling a scary story. These stories usually range from retelling of old folk stories to completely new stories.
The word “creepypasta” is a derivative of “copypasta” (copy-paste), which is usually an inflammatory or heart-touching short story that gets copy-and-pasted all over the place (e.g. forwarded emails). Creepypasta is the creepier version of copypasta, with short stories meant to scare you or just creep you out.
The SCP Foundation wiki is an interesting example of creepypasta. Each story in this collection is user-submitted and presented as though it was a scientific report of paranormal phenomena. Confused? Start here, with the original story that started it all.
The premise is a that a shadowy underground scientific facility has been collecting anything strange, otherworldly, supernatural, paranormal, or just plain weird.