there can be 2 definitions: one negative, the other positive...
1. a girl who has sex but does not enjoy it, who is used by guys and lets them do it. She does not have the self respect to make them stop.
2. a girl who likes sex, so she does it. She probably is not in a relationship, and she equalizes herself to guys by treating sex the same way they do.
The difference between the two examples of what society labels as "sluts" is that one has self respect, the other does not. No one should be called a slut anyway, especially when the name "slut" is a purely judgemental and stereotypical one, and no one really knows the real story behind it all, which is pretty much always the case.
"She has sex...therefore she must be a slut."
I completely disagree with that, but thats how society sees it.