Ipod library

I know this is probably a stupid question, but I've just recently bought an Ipod and instead of trying to figure out this for myself I figured I'd throw this question out there, because often it can be quicker to solve.

How do I make it so that I have more music on my Ipod than in my Itunes?. I know my ipod is in-sync (but with no n-sync.. get it?) with my itunes, but how do I make it so that I can add albums from my itunes library, then delete them from my itunes library, but still be able to keep them on my ipod next time I update it?

thanks for you help