The M1911 A1 is a recoil-operated, magazine-fed, self-loading hand weapon. It contains an inertia-type firing pin that makes it impossible for the firing pin to discharge or touch the primer, except on receiving an impact from the hammer. The action of the recoil spring forces the slide forward, feeding a live cartridge from the magazine into the chamber. The weapon is then ready to fire again.
The pistol is designed to fire calibre .45 cartridge ball ammunition and the magazine holds seven cartridges. The upper cartridge is stripped from the magazine and forced into the chamber by the forward motion of the slide.
On March 29th, 1911, the Browning-designed, Colt-produced .45 Automatic pistol, was selected as the official sidearm of the Armed Forces of U.S.A., and named Model 1911.
That original pistol was very similar to todays. One easily-distinguishable external difference is the crescent-shaped cuts behind the trigger of the contemporary pistols which were missing from the original design and were adopted on the model M1911A1. The differences can be seen here.

The Colt Model 1911 was slightly improved in early 1920's when the flat mainspring housing was replaced with an arched one, a shorter hammer spur was used, a short trigger was made standard as well as a longer safety grip. The new model was named Colt M-1911 A1 Government Model. In 1929, Colt also produced a 1911 pistol, based on the new .38 Super cartridge, while in 1931 a .22 LR version of the pistol was produced, named the Colt Ace. In this form the gun was produced in during the remaining years until WWII, when military requirements were met by production of the M-1911 by several firearms manufacturers such as Ithaca, Remington-Rand, Union Switch and Singer. Several thousand of these were produced during the war period.
After the war, the M-1911 was adopted by several armies around the world. Also, Colt signed contracts with some manufacturers in those countries, to produce this model. Here, you can see some pictures of an Argentinian M-1911.

In the 1980's Colt introduced a new series of all their models, with an additional safety device, namely a firing pin safety mechanism which didn't allow the pistol to fire if the trigger wasn't pulled to the end of its travel. The guns produced there after, are called Colt MKIV - Series 80. This safety system was deemed necessary in today's world of lawsuits but is rumoured to have a bad effect on trigger pull. This same firepin safety mechanism is also used in the high-capacity pistols produced by Para Ordnance.
During the nineties Colt announced their "Enhanced Series" of M-1911s, which were basically the Series 80 guns with several popular modifications. These modifications included a beavertail safety grip, beveled magazine well, flared ejection port and a small cut underneath the rear of the trigger guard, which allowed the pistol to sit lower in one's hand.
During the last two decades several other manufacturers started producing M-1911 pistols. Some of them just followed the traditional lines whilst others were 'state-of-the-art' based on polymer frames etc. One thing is clear, John Browning's design is still alive and doing extremely well after more than eight decades from its initial conception.