We started out with the XM1 and then the M1 was fielded. Based on feedback from the troops in the field, certain modifications were added to those initial M1s in the field and to M1s during production. These became known as M1IP or M1 Improved Production. This variant was equipped with the M68 105mm main gun and had the asymmetrical 3-blast panel configuration.
The M1A1 had two idenitcal turret blast panels and was upgunned to the 120mm smooth bore cannon. It also added an NBC over pressurization system in place of the left hull sponson (storage) box. A "man hole cover" was added forward of the loader's hatch in anticipation of the development of Commander's Independent Thermal Viewer (CIT-V). The CIT-V was not installed on any M1A1 though.
The next variant included changes the USMC required to make the Abrams suitable for amphibious operations. It also had the depleted uranium mesh added to the forward slopes of the turret and hull. Because these changes incorporated a heavier armor and was now a common USA/USMC tank, it was referred to as the M1A1 Heavy Common. The modeling world refers to this as the M1A1-HA for "heavy armor", but this nomenclature is not used in the real armor world.
The Marines made some cosmetic changes to their Abrams. They added a Missile Countermeasures Device (MCD) on top of the man hole cover. This is the square device that some people confuse for a version of the CIT-V. It serves a totally different purpose by directing anti-tank guided missiles away from the tank. The Marines do not use the CIT-V.
The M1A2 added the CIT-V which allows the commander to search and designate targets while the gunner engages the current target. This tank also incorporated an auxillary power unit (APU or basically a generator) in the turret bustle rack to power the optics and electronics without the need to run the turbine engine. The APU has been retrofitted to many M1A1s as well. The M1A2 also required a new commander's cupola because the CIT-V data screens took up too much room in the commander's hatch.
The final variant (so far) is the M1A2SEP or Systems Enhancement Program. This upgraded several internal systems and allowed the APU to be moved from the turret bustle to the inside of the left rear hull (Under Armor APU or UAAPU). The engine changed as well and the CIT-V now uses a 2nd generation FLIR.
Very few units in the Army use the M1A2SEP which are mainly remanufactured M1IPs. There are several additional rebuilt variants of the M1A1. One is called the M1A1-D and incorporates the digitized electronics package that is in the M1A2. Another is called the M1A1 AIM or Abrams Integrated Management Program which rebuilds older M1A1s to a zero time/zero hours "like new" condition.
Bunch of changes to a tank that has been in service for 20+ years. While the current tanks look like the same ones used during Desert Storm, they have a decade plus of modernization done to them.