The United States Colonial Marines (USCM) are a group of fictional space marines first introduced in the movie Aliens. They have taken a prominent role in the successive games, comics, and novels set in the Aliens series.
The actual details of the force are not expressly revealed in the movies. An associated book Aliens: Colonial Marines Technical Manual by Lee Brimmicombe-Wood was published by Boxtree in 1995.
The USCM force was created in 2101 as the United Americas Outer Rim Defense which was later succeeded by the force under its present name after the Torin Prime Civil War against J'Har Rebels.
Long after the first wars with the Xenomorphs in which they suffered many casualties, they were later succeeded by the United Systems Military.
The USCM are part of the United Americas Allied Command. The force has roughly 165,000 marines divided into four divisions, four aerospace wings, and numerous support groups. There is also a reserve division and aerospace wing.
The fighting groups are organized into three Marine Space Forces - Sol (two divisions), Eridani (one division), and Herculis (one division). Each has primary responsibility for security in their areas.
The Colonial Marines are likely best known for their use of the M41A pulse rifle, a futuristic assault rifle with an underslung grenade launcher, and for the M56 smart gun, a large machine gun which is actually worn by the user using a sophisticated harness mechanism. Featured in the movie Aliens are also the M240 flamethrower and the M4A3 pistol.
For more details see Weapons of the Colonial Marines.