The game takes place both before and after the events of Star Wars: A New Hope, and is played in the person of Kyle Katarn, a mercenary for hire. He is contacted by the Rebellion to stop a project from the Empire called the "Dark Trooper".
Often labeled a "Doom clone", it is believed that Dark Forces was created to counter the many unofficial Star Wars-themed WADs for Doom, and rumored that LucasArts reverse-engineered the Doom engine to find out how to build their own. The Dark Forces engine was in the end more advanced than the Doom engine, containing features such as rooms over rooms, polygonal objects, and the ability to look up and down.
Dark Forces was the 11th best selling computer game of the period 1993 to 1999, with 952,000 copies.
Official Lucasarts' Star Wars: Dark Forces web site (http://www.lucasarts.com/products/darkforces/splash.htm)
References
Turner, Benjamin & Bowen, Kevin (December 11, 2003). Bringin' in the DOOM Clones (http://archive.gamespy.com/articles/december03/doom/clones/index2.shtml) (page 2). GameSpy.