Here is some Wiki info:
The Tank Urban Survival Kit (TUSK) is a series of improvements to the M1 Abrams intended to improve fighting ability in urban environments.Historically, urban and other close battlefields have been the worst place for tanks to fight. A tank's front armor is much stronger than that on the sides, top, or rear. In an urban environment, attacks can come from any direction, and attackers can get close enough to reliably hit weak points in the tank's armor or gain sufficient elevation to hit the top armor.
Armor upgrades include reactive armor on the sides of the tank and slat armor (similar to that on the Stryker) on the rear to protect against rocket-propelled grenades and other shaped charge warheads. A Transparent Armor Gun Shield and a thermal sight system are added to the loader's top-mounted M240B 7.62 mm machine gun, and a Kongsberg Gruppen Remote Weapon Turret carrying a .50 caliber machine gun (again similar to that used on the Stryker) is in place of the tank commander's original .50 caliber machine gun mount, wherein the commander had to expose himself to fire the weapon manually. An exterior telephone allows supporting infantry to communicate with the tank commander.
The TUSK system is a field-installable kit that allows tanks to be upgraded without needing to be recalled to a maintenance depot. While the reactive armor may not be needed in most situations, like those present in maneuver warfare, items like the rear slat armor, loader's gun shield, infantry phone (which saw use on Marine Corps M1A1s as early as 2003), and Kongsberg Remote Weapons Station for the .50 in caliber machine gun will be added to the entire M1A2 fleet over time.
On 29 August 2006, General Dynamics Land Systems received a U.S. Army order for 505 Tank Urban Survivability Kits (TUSK) for Abrams main battle tanks supporting operations in Iraq, under a US$45 million contract. Deliveries were expected to be completed by April 2009.Under a separate order, the U.S. Army awarded General Dynamics Armament and Technical Products (GDATP) US$30 million to produce reactive armor kits to equip M1A2s. The reactive tiles for the M1 will be locally produced at GDATP's Burlington Technology Center. Tiles will be produced at the company's reactive armor facility in Stone County Operations, McHenry, Mississippi. On 8 December 2006, the U.S. Army added Counter Improvised Explosive Device enhancements to the M1A1 and M1A2 TUSK, awarding GDLS $11.3 million contract, part of the $59 million package mentioned above. In December, GDLS also received an order, amounting to around 40% of a US$48 million order, for loader's thermal weapon sights being part of the TUSK system improvements for the M1A1 and M1A2 Abrams Tanks.