A spoiler is an important part of a plot that when given away to someone who has yet to experience the movie, show, game, etc. may 'spoil' or ruin said experience.
Example
John, who has yet to see The Empire Strikes Back, is speaking to Fred who has just seen it yesterday..
"So Fred, how was the new Star Wars film?" - John
"It was f**king awesome!! Darth Vader is totally Luke's dad, and Leia is his sister, then Vader completely cut his friggin' hand off, but now he has a cyborg hand." - Fred
"F**ck you, you son of a bitch, you just spoiled the movie I was looking forward to seeing" - John
Spoilers as Entertainment
A few television shows, morning zoos, and obnoxious, but funny, jerks have taken it upon themselves to spoil movies and books for nerds who line up for days dressed in costumes. Of course many of them get upset, some yell, others cry, but in the end they get what they deserve.
