Why do we need to use assemblies ?

Assemblies can contains the information about Metadata,resources,menifest,etc....

When we build .net application assemblies are created in terms of .dll or exe. When we build a class and bulk it it creates a .dll file . Public key token gives the uniqueness of the assembly . Here is a good read t understand http://stackoverflow.com/questions/1321419/what-is-the-role-of-public-key-token