From what I know they are contractors (mercenaries), that means the guys are ex-soldiers, Special Forces etc. (SeAL's, Marines, Rangers, etc. ) that contracts to provide services of a military nature! The Blackwater was founded by a Navy SEAL in 1997 and now I think they recruit from all over the world, even here in Romania, the new owner of Romanian Special Forces in Afghanistan is Blackwater.
Academi is the most powerful mercenary company on the planet with 20,000 troops, a fleet of 30 aircraft, many combat helicopters and even a private intelligence division. Blackwater was chopped by the media and public opinion because of Nisour Square massacre in Baghdad, which killed over 20 people, including women and children. The incident prompted the company to change its name.
Despite all these claims, the Pentagon plans to keep 10,000 troops (they say) combating terrorism and maintaining stability in an unpredictable future.
*Almost 1 ½ year ago I watched a documentary about a big flux of ex-Romanian soldiers who had combat tours in Iraq or Afghanistan and who chose after that to work for Blackwater as security guards in Iraq ! From watching that documentary I have this information.