Often called the M1911 or just 1911, it's a semi-automatic handgun originally designed by John M. Browning that was widely used by the US military since World War I. Due to its simple design and dependability it was later adopted by law enforcement, civilians and gangsters alike with most major gun manufacturers around the world producing their own version of the 1911 design.
The weapon has a magazine capacity of 7 rounds and the player can carry an additional 4 magazines for a total of 35 rounds. It has a semi-automatic rate of fire with a low amount of recoil, making it extremely accurate.
You can find more real world information about the Colt 1911 on its Wikipedia page.