Headhunters are a special profession... unlike Assassins, Headhunters are not necessarily trained to eliminate people. They are trained to make people disappear, dead or alive. And that is a very important distinction. Assassins make an art out of making people die in all sorts of ways. Headhunters make an art out of transporting individuals from one location to another, dead or alive, depending on the task. Big difference.
While Assassins almost exclusively rely on stealth and raw strength to make people die, Headhunters have to be even sneaker and more vigilant, as sometimes they have to deliver people alive to the task-givers, or have to deliver whole corpses hopefully unscathed. That is where the name "Headhunter" comes from - they "hunt" for "heads" (individuals), and bring them back "home" (to the person who gave the task).
The inventory of a headhunter's methods can include just about anything: not just stealth and physical prowess, but also magic, seduction, deceit, and all sorts of dirty tricks only the most deviant can come up with.
Headhunters are a prestige class available to all classes but Clerics and Druids.
Being a Headhunter grants Advanced Stealth and a unique ability named Anaesthesia, an undefined "headhunter trick" that knocks people unconscious for 1-2 hours. It's exact nature has never been defined, but it is implied to be either a poison or a physical attack that is non-lethal, but still strong enough to knock someone unconscious.
Headhunters do not make an appearence in the 2012 and 2013 video game adaptations, but are planned to be included in the 3D game.