Other forms: whores; whoring; whored
In the original sense, a whore is a prostitute: someone who has sex for money. The word has since broadened to mean anyone who is money-hungry.
Whores traditionally whored their bodies out for money. But today a whore can be a person that compromises himself in any kind of way for monetary gain. It's also a nasty thing to call someone (especially a woman) who has a lot of sex and is often used as a vulgar insult similar to bitch. It's not really an appropriate word to call someone, including actual prostitutes.