What is GAC?

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. Assemblies deployed in the global assembly
cache must have a strong name. When an assembly is added to the
global assembly cache, integrity checks are performed on all files
that make up the assembly. The cache performs these integrity checks
to ensure that an assembly has not been tampered with, for example,
when a file has changed but the manifest does not reflect the
change. Use a developer tool called the Global Assembly Cache tool
(Gacutil.exe), provided by the .NET Framework SDK or Use Windows
Explorer to drag assemblies into the cache. To install a strong-
named assembly into the global assembly cache At the command prompt,
type the following command:
gacutil I
In this command, assembly name is the name of the assembly to
install in the global assembly cache.