Each computer on which the Common Language Runtime is installed has a machine-wide code cache called the 'Global Assembly Cache'. The Global Assembly Cache (GAC) enables you to share assemblies across numerous applications.
The GAC is automatically installed with the .NET runtime. The global assembly cache is located in 'Windows/WinNT' directory and inherits the directory's access control list that administrators have used to protect the folder.
The approach of having a specially controlled central repository addresses the shared library concept and helps to avoid pitfalls of other solutions that lead to drawbacks like DLL hell.
The Global Assembly Cache Tool (Gacutil.exe), that allows you to view and manipulate the contents of the Global Assembly Cache.