By default the GAC folder in Windows is located at %windir%\assembly, you can find all the registered DLLs in that folder. Whilst you can open it in windows explorer and view it like any other folder, it is a somewhat special and allows you to have different versions of the same DLL registered in the GAC.
Open up DOS prompt and navigate to the GAC folder, for instance:
and you can see that the GAC is actually a folder inside the assembly folder, and drilling a little deeper reveals that each DLL has its own folder which contains all the registered versions, each as a folder that contains the actual DLL inside:
.Net 4
For .Net 4, the GAC location is now %windir%\Microsoft.Net\assembly.
Global Assembly Cache Tool
You can use gacutil.exe from the command line to view (e.g. gacutil /l), add (e.g. gacutil /i SomeAssembly.dll) or remove (e.g. gacutil /u SomeAssembly) the contents of the GAC, this of course, supports multiple versions of the same assembly too.