The GAC holds assemblies installed on the local host that are available for shared use by other code. The Zap Cache is a sub-cache of the GAC that holds assemblies that have been precompiled into native machine code. Zap Cache assemblies are typically Fully Trusted libraries that are used frequently by the CLR itself, so precompilation boosts performance.

If you would like to know more, take a look at http://www.nsa.gov/ia/_files/app/oldFiles/NET_Framework_Sec1.pdf