I generally agree with Gaborn; I had the general concept already defined in my head, but he broadened it and synthesized it very well.
A "slut" would be a person who engages in sexual behavior for reasons we consider unhealthy or inappropriate. Appropriate, healthy sexual relations include sex for pleasure, for love, or for bonding. That list is not intended to be exhaustive.
Other motivations for sex may include using sex as currency, or as a means to boost self esteem. These are less healthy. Again, that list is not exhaustive, but represent what are, to me, unhealthy reasons to have sex.