In my opinion, a slut is someone who longs for, lusts, needs and/or can't get the thought of sexual relations or acts out of their heads. They will go to extreems to fulfil their sexual needs. The more of their time such thoughts dominate their minds, the more of a slut they are.

I have also found that many people, in my opinion, over use the word slut, inserting it into the end (or beginning) of every statement during a scene. For me, name calling starts to lose it's significance when it is used as punctuation.