It's a movement not just to relaim the word, but as Boston organiser Siobhan Connors says, "is in protest of a culture that we think is too permissive when it comes to rape and sexual assault. It's to bring awareness to the shame and degradation women still face for expressing their sexuality... essentially for behaving in a healthy and sexual way."
It came about as a result of a senior police officer in Toronto advising female students to "avoid dressing like sluts in order not to be victimised." (Yes, he's since been disciplined).
I'll hold off on a long moral rant about how something like SlutWalk shouldn't even be necessary in today's society, because evidently it still is.
Links: BBC website report and SlutWalk Toronto Blog Page.