Code Access Security Primer

I was trying to explain quickly what Code Access Security is to someone when I found this blog entry from Emmet Gray:
http://home.hot.rr.com/graye/Articles/CodeAccessSecurity.htm

In a nutshell, he explains why your code won't run if run from a network share and what are CAS, mscorcfg.msc and caspol.exe.  It's a great 5 minutes CAS primer.