It was a word used to belittle women who were seen (by men) as too promiscuous.
It is now a word that women have taken back and you can use slut as a compliment. E.g 'wow you look like a slut' 'Oh thanks, my slutty dress has pockets' 
So the word can still be hurled around but means absolutely nothing. It says more about the person who said it than the person it's said about.
E.g if a man calls a woman a slut. Mentally healthy people know that the man is just a cunt and the woman is amazing.