What is a slut?

I know that this is a topic that is often discussed, but I would like to know what a slut is, and what defines a slut, and why are females more apt to be considered sluts than males? If a girl sleeps around, or has a relatively large number, say around 5, then she is considered a slut and people talk about her like she is a bad person. Even her friends whose number is less will talk about her behind her back and make rash judgments about her character as a person. However, if a male has sex with about 5 people that isn't that much and he gets judged for having such a small number. The more women a man has sex with, the better and the cooler he is as a person. If a woman has the same number as a man she will looked down on more so than the male, because it is inappropriate behavior for a female to behave in such a manner.
It is a double standard, and a double standard that has been around for a while. It is really hard to break it because sex is praised more in the male world than the female world. It is hard for women to enjoy having sex without being branded a slut. If she does want to have sex she should only have sex with one person whom she is in a serious relationship with. It is not fair and aggravating to hear my male friends get praised for their "conquests", but my female friends get insulted and judged for having random sex.