Slut is a slang term used to indicate a person who has frequent sexual relationships with different people, especially short-term affairs. It is also applied to people who are promiscuous in that they have, or are reputed to have, many sexual partners, or whose sexuality is voracious, indiscriminate, and shameful.
In society at large, this term is generally regarded as at least mildly offensive, especially when applied to someone other than oneself. The term is usually applied to women (and can be compared to the more negative slag in British English), but is also often used in the gay male and bisexual communities. A longstanding joke explains that "a slut is a woman with the morals of a man."
In recent decades, especially in the gay, bisexual, and polyamorous communities, there has been an effort to reclaim the word. Especially among gays and bisexuals it may be used by the person concerned as an expression of pride in their status, or by others as an expression of envy of their "success rate".
The book The Ethical Slut brought wide public awareness of this reclaimed usage among polyamorous and non-monogamous people, as an expression of choice to openly have multiple partners, and revel in that choice: "A slut is a person of any gender who has the courage to lead life according to the radical proposition that sex is nice and pleasure is good for you".
Probably because "slut" has never had any particular connotation of doing it for money, it has been far more reclaimed in this sense than the word "whore". Many who would gladly say "I'm a slut" would be very offended to be called a "whore". This distinction does not exist in most of the Romance languages: in Spanish, for example, a puta is equally a "slut" or a "whore", and there has been far less reclaiming of the word.