"Armored Core is a mecha-based video game series developed by From Software for the PlayStation Portable, PlayStation, PlayStation 2, PlayStation 3, Xbox 360 and Mobile Phone platforms. Armored Core for Answer, the thirteenth title in the series was released in Japan on March 19, 2008.
Armored Core games are Mecha Simulation / third-person shooters, in which the player pilots a large mecha called an Armored Core, or AC for short. The name Core comes from the unlimited endurance Power-Generators Technology used recently among the different nations, in conjunction with a practically unlimited variety of Weapons, and Vehicle Propulsion-systems, manufactured by several different companies.
The player builds their AC from parts that can be bought from a shop, found in missions, or by fulfilling certain requirements. Money is earned by performing missions as a mercenary, giving the player some choice in which missions to engage in.
Typically, in each game, the player is a 'Raven', a member of a mercenary organization, and is hired by corporations to do various missions, then after the mission, they are charged for repairs and ammunition, after which one can continue on with missions or customize their AC."
Source: Wikipedia, "Armored_Core", available under the CC-BY-SA License.