Spoilers are little nuggets of information that give away the twist.
Every good twist ending leads inevitably to spoilers. Someone wants to share that miraculous "Wow! Who knew?!" Moment, and when that moment is coupled with poor judgement, or a complete lack of empathy, a spolier is born (and a dick).
Seriously, if you know the ending of say, Memento or The Others, keep it to yourself. Unless you are writing a topic page on Spoilers, in which case this page will be loaded with them, so don't complain, you've been fucking warned.