The Global Assembly Cache (GAC)

The Global Assembly Cache (GAC)

The Global Assembly Cache (GAC) is a central repository for storing shared assemblies. The GAC allows multiple versions of the same assembly to be installed concurrently and also prevents different assembly vendors from overwriting each other's assemblies.
The use of the GAC is encouraged for assemblies that will be used by more than one application on the system, and also for assemblies that are expected to be versioned in the future. When an application is launched, Mono extracts the name of the assembly, the version and its public key token and loads the required assembly from the GAC.
The GAC is a black boxed repository of assemblies capable of keeping multiple version/cultures of a same assembly. A strongname is considered a unique entry in the GAC (i.e. the GAC requires all it's assemblies to be strongnamed and signed). Note that the GAC has nothing to do with security! In fact assemblies in the GAC aren't verified when loaded from the runtime. The GAC is for manageability (avoiding DLL hell).

Source: http://www.mono-project.com/Assemblies_and_the_GAC