The Atlas, first tested in 1959, was the United States' first successful ICBM (Intercontinental Ballistic Missile). It was a multistage, liquid-fueled (liquid oxygen and kerosene) rocket, with three engines which produced 162,000kg of thrust.
Though never used in combat, it was used as the expendable launch system for the Mariner space probes used to study Mercury, Venus, and Mars (1962-1973); and to launch all but the first two Mercury program missions (1962-1963). The Mercury-Atlas missions resulted in the first American to orbit the earth (Lt. Col. John H. Glenn Jr) in February of 1962. (Major Yuri A. Gagarin, a Soviet cosmonaut, was the first human to orbit in April of 1961.) Atlas launched the Agena Target Vehicles used during the Gemini program. Direct Atlas descendants continue to be used as satellite launch vehicles to this day.
Atlas was suggested for use by the United States Air Force in what became known as Project Vanguard. This suggestion was ultimately turned down, however, as Atlas would not be operational in time and was seen by many as being too heavily connected to the military for use in the US's IGY satellite attempt.
Atlas got its start in 1946 with the award of a Army Air Forces research contract to Convair for the study of a 1500-5000 mile range nuclear armed missile. This was the MX-774 or Hiroc project. The contract was canceled in 1947 but the Army Air Forces allowed Convair to launch the three almost-completed research vehicles using the remaining contract funds. The three flights were only partially successful. However they did show that balloon tanks, and gimballed rocket engines were valid concepts.
Atlas is almost unique in its use of balloon tanks for fuel, made of very thin stainless steel with minimal or no rigid support structures. Pressure in the tanks privides the structural rigidity required for flight. An Atlas rocket will collapse under its own weight if not kept pressurized. The only other known use of balloon tanks at the time of writing is the Centaur high-energy upper-stage.
Atlas is further unique due to its odd staging system. Most rockets stage by dropping both engines and fuel tanks. Atlas drops just two of its three engines and no fuel tanks. Rockets using this technique are sometimes called stage and a half boosters. This technique is made possible by the extreme light weight of the balloon tanks. The tanks make up such a small percentage of the total booster weight that the weight penalty of lifting them to orbit is not offset by the technical and weight penalty required to throw half of them away mid-flight. Depending on how you look at it, this makes Atlas a Single stage to orbit booster (though most call it a 1.5 stage to orbit).
The newest version of Atlas, the Atlas V, no longer uses balloon tanks nor 1.5 staging, but incorporates a rigid framework for it's first stage booster much like the Titan family of vehicles. The Atlas V is an Atlas in name alone as it contains little Atlas technology.