The global assembly cache (GAC) is a machine-wide code cache that stores assemblies specifically designated to be shared by several applications on the computer.
The Global Assembly Cache in the .NET framework provides a central place for registering assemblies. These assemblies once registered are then available to all applications for usage.
Global assembly cache is nothing but a special disk folder where all the shared assemblies will be kept. It is located under <drive>:\WinNT\Assembly folder.