Mercenaries, sometimes called "mercs", are freelancing soldiers for hire. They generally have no real allegiances except to widgets; thus, their loyalties can change for the right price.
Though mercenaries' general reputation is of changing sides simply because "the other guy made a better offer," mercenaries can have personal codes of morality and ethics that vary wildly from one to another. Where one mercenary might stab his employer in the back the minute he receives a better offer, another may wait until the contract is fufilled before taking on a job to off him. Others will simply never take a job against a former employer because "repeat customers can't buy services if they're dead."
Jobs for mercenaries can vary widely; from bounty hunting to simple wreaking havoc, as long as the price is right (and it doesn't break their moral code, if they have one), they'll do it.
Unlike pirates, mercenaries operate both inside and outside the law depending on their moral codes and reputation. They're just as likely to join a good cause as a bad one, so they may be welcomed into a village with open arms, or hunted down by the local military as soon as they step foot over the border. Every mercenary is different.