In short: The Bounty Hunter is a mercenary that captures fugitives for the price on their head. The thief can create some very clever traps, especially at the higher levels. One of them even emulates the Imprison (Maze?) spell. He may use any weapon, not just weapons allowed to normal thieves, but cannot be of lawful alignment.
Requirements: Ability scores of at least 11 in all skills, except charisma
Description: The Bounty Hunter is a ruthless mercenary, worshipping little besides the price on his target's head, recognizing few laws save the contractual distinction between "kill" and "capture". He may be found serving the state, capturing criminals and bringing them to justice; or he may serve the shadowy lords of the underworld, avenging the twisted honour found amongst thieves and criminals. Pursuit of his quarry may take him through a thousand hostile environments, to foreign lands, even to alien planes. A further requirement is that the Bounty Hunter be of non lawful alignment.
It is important to draw a distinction between the Bounty Hunter and the Assassin, for their vocations are similar. The Assassin is most often part of a larger network or organization - either a society of Assassins for hire, or a guild or crime family, or even a government. The Assassin is retained by that organization to discreetly eliminate its enemies; he is strictly a killer. The Assassin is also a predominantly urban figure, though his missions may take him out of that setting on occasion. Most organizations that have assassins would deny their existence, because of the highly illegal and unpopular nature of their activities.
The Bounty Hunter, by contrast, is a loner. He may be solicited directly by an employer, but more often he simply learns of a price offered for the body (living or dead) of some person and goes after him.
While the Assassin requires secrecy and anonymity, the Bounty Hunter thrives on infamy. Fear leads his prey to make mistakes, and each such mistake brings the Bounty Hunter one step closer to success. While an Assassin is often hired to kill relatively normal, often unsuspecting people, the Bounty Hunter is tracking fugitives - people who know who's after them, and are often exceptionally desperate and dangerous.
Pursuit of such people may lead the Bounty Hunter to literally any place, even to other planes of existence (if the prospective reward will make the venture worthwhile) and so the Bounty Hunter becomes adapt at survival, and tracking, in all manners of hostile environments.
Bounty Hunters do not track only fugitives. They may be hired to perform such tasks as kidnappings, freeing kidnapped persons, or (especially at lower levels, when they're still developing their skills) recovering stolen property.
The Law and authorities do not always look kindly upon Bounty Hunters, though they will permit their existence so that they too, may benefit from the manhunter's experience. For the same reason, thieves' guilds tolerate the Bounty Hunter, despite the fact that almost no Hunter would ever join their ranks.
Weapon Proficiencies: The Bounty Hunter is permitted the use of any weapon. As part of his persona and fearsome public image, a Bounty Hunter will often gain proficiency in a rare or bizarre weapon, such as the khopesh sword or man-catcher. Non-thief weapons take up two of the Bounty Hunter's weapon proficiencies slots, but he is granted a bonus slot at 1st level.
Armor/Equipment: Besides the usual range of thiefly equipment. Bounty Hunters take interest in items for killing and capturing their prey. Special items from the equipment chapter, such as blade boots, death knives, folding bows, and the like, are sometimes taken as favourite weapons. A rope for holding live prisoners is, of course, vital and it may also be used for setting snares. Blinding powder and incapacitating poisons (paralytic ones or those that make their victim ill and helpless) may also have value.
Bounty Hunters make little use of deadly poisons - that is more the province of the stealthy Assassin. If a Bounty Hunter is out to kill a fugitive, he probably won't be worrying about how messy it will be.
Bounty Hunters gain no special benefits nor any special hindrances.
Source: The Complete Thief's Handbook