GAC is known as Global Assembly Cache where the shared assembiles are being kept. GAC can be found on a system at \\Assembly ie. C:\Windows\Assembly.
GAC is a Folder that contains DLL that have strong name. As DLL differ by strong names+public key and since strong name can not be same hence we can put two DLL with same name into GAC. For example SameNameApp.dll and SameNameApp.dll can co-exist in GAC if the first one have version 188.8.131.52 and the second have 184.108.40.206.
DLL =Assembly_name + Version + Culture_Info + public_key