Global Assembly Cache
Abbreviated as GAC, the Global Assembly Cache is a machine-wide store used to hold assemblies that are intended to be shared by several applications on the machine. Each computer where the common language runtime (CLR) is installed has a global assembly cache. The global assembly cache stores the assemblies specifically designated to be shared by several applications on the computer.
The .NET Framework provides two tools for working with the cache. One is a Windows shell extension that allows you to work with the cache using a Graphical User Interface (GUI). The other is a command line tool, called the Global Assembly Cache tool (Gacutil.exe), that is typically used in build and test scripts.