GAMS stands for General Algebraic Modeling System and automates the process of going from a mathematical statement of the problem to the solution. GAMS transforms the mathematical representation to representations required by specific solver engines like CONOPT, MINOS, CPLEX,..
GAMS consists of a language compiler, a stable of integrated high-performance solvers, and a rich set of connectivity and productivity tools.
GAMS is tailored for complex, large scale modeling applications, and allows you to build large maintainable models that can be adapted quickly to new situations. A general overview about GAMS is available at: http://www.gams.com/presentations/present_softwaredemo2.pdf
There are also some entries in Wikipedia (http://en.wikipedia.org):
The GAMS web sites contain much more (technical) information about GAMS: