UT3 Level streaming Thread

Few questions, say I want to make a ridiculously large UT3 map. I would use streaming to do this and prevent loss of performance right? What exactly does this do and how does it work? (Don't need help on how to do it so much, there are tutorials on it that I'm going to read up) I just want to know what exactly streaming does and how it is applied to levels.

I've got the perfect link for you:
Hourences.com

Hourences has a great stash of tutorials on all aspects of the U3 engine. Hopfully this helps ya out!