Help understanding the GAC_MSIL directory

hi there,

I have a Class Library Project that makes a reference to a dll

when I right click the reference I see this path for the DLL:

C:\WINDOWS\assembly\GAC_MSIL\Microsoft.Practices.EnterpriseLibrary.Logging\2.0.0.0__bd5a587c51257568\Microsoft.Practices.EnterpriseLibrary.Logging.dll

Now, If I creeate a new project and open the Add reference wizard, this dll doesn't appear on the .NET tab.

If I copy the path:

C:\WINDOWS\assembly\GAC_MSIL\Microsoft.Practices.EnterpriseLibrary.Logging\2.0.0.0__bd5a587c51257568\ 

 above in the execute box,  I can manually copy the DLL to some other folder and browse to it.

That works but I feel I'm doing something wrong and I'm not getting some concept behind that...

Please help me understand the C:\WINDOWS\assembly\GAC_MSIL directory.

It's a strange directory because it doesn't appear on the windows explorer if I try to browse ...