What is GAC and why we need GAC ?

Question

If the application has to be shared among several application.
 If the assembly has some special security requirements like only administrators
can remove the assembly.If the assembly is private then a simple delete of
assembly the assembly file will remove the assembly.
Note :- Registering .NET assembly in GAC can lead to the old problem of DLL hell.
Where COM version was stored in central registry.So GAC should be used when
absolutely necessary.