A whore is a woman who engages in sexual intercourse for money, a prostitute.
Some women enjoy the notoriety and thrill of being called, or labelling themselves, a whore. What they normally mean to convey is that they enjoy sexual looseness and freedom; they are highly sexed; they enjoy being seen as a sexually fulfilling woman and like to be used by men (or perhaps just one man) for sexual gratification.
People using the word in this manner would not necessarily be happy if money were paid for their use, though they may consider it their dominant's right to hire them out.
The word itself has a complex history. The prototypic root appears to be *k-, "to like, desire" (hence the Dutch 'graag' meaning 'YES PLEASE!'). The derived stem is *kro-, origin of the prehistoric Common Germanic word *hraz meaning "one who desires" (often "adulterer"). The feminine of this, *hrn-, becomes hre in Old English, a possible ancestor of Modern English 'whore'. On the other hand, we have the Dutch 'huur' commonly meaning 'hire' as in 'te huur' - 'for rent' and seen on estate-agents signs all over Amsterdam. 'Salford van hire' would be 'Zaalvoord auto huuren' in Dutch. Dutch sailors have spread much of their language around the world ('gangplank', starbord', 'on-board', 'underway', 'scouse')