Permission set is a collection of permissions which are 
used to modify the security policy for code groups. The CLR 
provides the following built-in permission sets. 

Nothing 
Implies that the code cannot be executed. 

Execution 
Implies that code does not have permission to access 
protected resources, but can execute. 

Internet 
Implies that code downloaded from a location and has 
limited permissions. 

LocalIntranet 
Implies that the code holds permissions defined in the 
default enterprise policy. 

Everything 
Implies that the code has all permissions except to skip 
verification. 

FullTrust 
Code has full access to all system resources.