A bounty hunter was a person who hunted, captured or killed people and/or creatures for money. Most bounty hunters worked for themselves, taking contracts offered up by various folks, ranging from Supreme Chancellors and Senators to Hutt crime lords and Black Sun Vigos. Some bounty hunters were ethical practitioners of their trade, such as Mira, while others, such as her nemesis Hanharr, were bloodthirsty, bipedal slaughter-factories.
Even droids could be bounty hunters. HK-47 suffered the indignity of being reduced to a common bounty hunter (as opposed to a well-crafted assassin droid with unrivaled sophistication) by at least one of his former, non-Revan masters. IG-88, on the other hand, enjoyed bounty hunting rather well. Of course, the difference between the two professions was a very thin distinction, and one could often be confused with the other.
Bounty hunters were considered by many to be the scum of the universe. They did not possess any marketable skills that could secure them a socially-acceptable career, so they sold their services to the highest bidder. This practice essentially made them blaster-toting prostitutes.