A slut is someone who has sex with people to get things -- attention, gifts, etc. You don't even need to have sex with a lot of people to be a slut -- if you just give head for your nifty jewelry or to be popular, then hey... However, if you are doing it just because you enjoy sex, I don't care how many partners you have, I wouldn't call you a slut... Possibly high risk for disease, depending on who & how many, but not a slut...
I've heard many definitions of the word "slut", but Dawn_Sky has stated mine exactly. It's a mentality, and can refer to either sex IMO.