Creating Shared Assemblies
Usually, assemblies are private, which means they are meant to be used by only one application. But you might have a DLL intended for use by several applications, in which case you could make it shared.
Shared assemblies are stored in the Global Assembly Cache (GAC), and you'll find familiar assemblies like System and System.Data there. Assemblies in the GAC are usually DLLs, and applications share access to these assemblies. Note that you should not place assemblies in the GAC unless there is a pressing reason to do so, and you're using those assemblies in multiple applications. It's a very bad idea to fill up the GAC for no good reason.
You can see the GAC in several ways. You can select Start, Programs, Administrative ...