Slut is a pejorative term for a person (usually female) who is more sexually promiscuous than is deemed socially acceptable. The term has traditionally been applied to women and is generally used as an insult or offensive term of disparagement. Slut has also been reclaimed as a slang term in the BDSM, polyamorous and gay and bisexual communities. It may be used by the person concerned as an expression of pride in their status, or as an expression of enviousness in the "success rate" of others. The term is not interchangeable with whore or prostitute as those terms denote a person who engages in sex for money.
With the growth of the internet and the growth of internet porn the definition of slut has probably changed some. It is still reserved for women who have had the most sexual partners. I took some classes at a community college recently and several of the guys (ages 18 - 21) knew a girl who appeared on internet porn sites. When I was their age, if a girl was involved in porn it was a closely held secret. This might be giving away too much, but Miss November, Donna Edmondson, went to my high school. I never knew her and she graduated a couple of years before me, but it's kinda cool to think we walked the same halls and sat in the same classrooms.
It bothers me when people call someone a slut. I have found that women who some would consider to be a slut are actually some of the sweetest most giving people you will ever meet. If they have a flaw it is being too needy. I think promiscuous women are looking for acceptance anywhere they can find it. They need our love not our condemnation.
I love the scene in Biloxi Blues when Matthew Broderick's character admits to the prostitute that he is a virgin. She is very tender and caring. I don't know how representative of all prostitutes she is, but I like to think that they can be kind. If it wasn't for "sluts" I know I would never have gotten laid.