This is probably going to be one of those obvious when you think about it questions .

I have downloaded the Microsoft Enterprise Library and am reading the docs.

I the section Building the Enterprise Library under Preparing for deployment it says.

Alternatively, the Enterprise Library application block assemblies can be signed with a strong name key. This will ensure that their names are globally unique and will also provide versioning. Therefore, assemblies can be deployed in a shared location and used by multiple applications. A typical situation is that you deploy Enterprise Library application block assemblies in the global assembly cache. This allows all applications on the computer to use the Enterprise Library.
So what does it mean to sign with a strong name key?
Where is the global assembly cache?