Shared Assemblies and Strongly Named Assemblies(Chapter 3 of CLR via C#)

If an assembly is to be accessed by multiple applications, the assembly must be placed into a well-known directory, and the CLR must know to look in this directory automatically when a reference to the assembly is detected. This well-known location is called the global assembly

cache (GAC), which can usually be found in the following directory (assuming that Windows is installed in the C:\Windows directory): C:\Windows\Assembly

The GAC directory is structured: It contains many subdirectories, and an algorithm is used to generate the names of these subdirectories. You should never manually copy assembly files into the GAC; instead, you should use tools to accomplish this task. These tools know the GAC’s internal structure and how to generate the proper subdirectory names.