OPFOR is military-speak. It is an acronym for OPposing FORce(s). It is used most frequent in exercises and simulations to refer to those units representing 'the enemy.'

There is also a real-world equivalent. At the U.S. National Training Center at Fort Irwin, CA there is an army unit known jokingly as "the best-trained Soviet Motorized Rifle Battalion in the world." It is a group of permanently-stationed soldiers who are trained in the use of 'enemy' methods and equipment, and serve as said 'enemy' for every U.S. and allied unit that passes through the NTC. This matchup is also sometimes called the Kobayashi Maru, because it is nearly impossible to beat the OPFOR. This makes sense; they see an order of magnitude more (simulated) combat than any other unit, and they're always fighting on their home turf - a very familiar piece of California desert ground. As such, young officers who come up against the OPFOR are watched not so much to see if they win but to determine how well they keep their cool and remember/apply their training when the situation begins to go against them (and it will, young Jedi - it will).

The OPFOR unit is, in 'reality,' the 11th ACR or Armored Cavalry Regiment. They maintain a full set of equipment for the job, both 'actual' (T-80, T-72 tanks, Russian BTR, etc) and mocked-up American gear called VISMOD for Visual Modifications. They maintain a training program called the OPFOR Academy, which introduces new hands not only to the actual operations of the NTC and 'real' emergency procedures, but also indoctrinates (heh) them in the conflict which they are modeling. The nations of The People's Democratic Republic of Krasnovia, the Kingdom of Mojave and the Pahrumpian People's Guerrilla forces will all see combat during an NTC 'rotation' - and they'll tell you why and how.