1. When talking about a movie to an audience that has not yet seen the movie, this phrase is used by a speaker to warn that they are about to reveal plot details that may be surprising or unexpected. The thought is that revealing certain plot details will detract from the natural viewing experience, so the warning serves as an opportunity for people to stop listening/reading.
1a. The same idea, but in the context of other media such as books, television shows, etc.
2. Often this phrase is used jokingly following, or immediately proceeding information that is felt to be obvious.
1) Spoiler alert! We're about to discuss the ending of the Empire Strikes Back, so stop reading now if you haven't seen it yet.
2) "I loved Titanic, the best part was the special effects during the ship sinking... oh sorry, spoiler alert."
A girl (typically a crazed fangirl) who cannot get enough of her particular brand of show crack (ie merlin,grey's anatomy, etc) And spends most of her waking hours(or tries to at least), despite the fact that she has a REAL life and has things to do; looking for any possible info regarding said show. Specifically on forums where she can get her daily dose of spoilage.
Typically spoiler whores are people (girls) who can never get enough info and when we do we are forever speculating and eventually end up writing or reading fanfic to get our satisfaction. :)
-Hey Brooke, do you know the epic frepicness you are in store for on this episode of Merlin?
-No, I just know from the promo at the end of the show.
-Giiiiirrrlllll, let me tell you(commences to tell)
-How did you find all that stuff out?!?
-(sigh) Im such a spoiler whore...i just HAFTA know whats gunna happen!!!