.45 Auto generally refers to the Model 1911 .45 caliber auto loading pistol.
Origionally designed by John Moses Browning and marketed to the U.S. Army for use by the Cavalry. The 1911 is a single-action,semi-automatic, gas blowback operated, magazine-fed pistol. It fires a 230 grain, .45ACP bullet at speeds of less than 1000 feet per second. Generally the magazine holds 7 rounds of ammunition, but larger capacities are available.
The 230 Grain slug was actually designed to stop horses in mounted cavalry battle. The 230 grain slug moves slowly, but gives up 100% of its delivered energy into 6" of tissue.
Although the 1911 was replaced by the Baretta Model 92 chambered in the less effective 9mm, the U.S. Marine Corp as well as the Special Operations community have returned it to service.
The 1911 has seen many changes over the years although no signifigant mechanical changes to its overall design. There was the release of a Colt "Series 80" model that unneccessarily included a firing pin block in case a loaded pistol was dropped. This additional safety was later discontinued. The ejection port was also lowered and flared to facilitate better ejection of spent casings.
The 1911 has undergone a rennasiance recently. Without the need for high-performance, or gimmick ammunition, the .45ACP cartridge remains one of the most effective manstopping rounds available.
Several companies make clones of this classic piece, although you do get what you pay for.
Among the higher end clones are Kimber, Les Baer, Wilson Combat, Double Star, Para-Ordinance, Detonics, and George Heinie.
Mid-Market quality clones include Springield Armory (Though they make a higher quality clone too), Rock Island Armory, Charles Daily (RI and CD are both produced by Twin Pines in the Phillipines) and Norinco of China (Yes China makes a great clone, though they are no longer imported and hard to find).
Low-end makers are Century Arms (Made by Shooters Armory) and Llama (now out of business).
The design of the 1911 is simple enough so that with a well made frame, money spent on parts, and some time with basic hand tools, all but the cheapest of the 1911 clones can be brought up to better than acceptable accuracy.
Colt still produces a version of the 1911, but they are expensive, and most are ordered through their Custom Shop with up to a 6-month waiting list.
I love my 1911 .45 Auto (not in the Biblical sense, but almost).
Feel safe at night, sleep with a .45 caliber teddy-bear.