What is GAC

Each computer where the common language runtime is installed has a machine-wide code cache called the global assembly cache.The global assembly cache stores assemblies specifically designated to be shared by several applications on the computer.

You should share assemblies by installing them into the global assembly cache only when you need to.

There are several ways to deploy an assembly into the global assembly cache:

• Use an installer designed to work with the global assembly cache. This is the preferred option for installing assemblies into the global assembly cache.
• Use a developer tool called the Global Assembly Cache tool (Gacutil.exe), provided by the .NET Framework SDK.
• Use Windows Explorer to drag assemblies into the cache.

There are scenarios where you explicitly do not want to install an assembly into the global assembly cache. If you place one of the assemblies that make up an application in the global assembly cache, you can no longer replicate or install the application by using the xcopy command to copy the application directory. You must move the assembly in the global assembly cache as well.