I just don't understand updater script and the framework-res.apk???

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http://forum.xda-developers.com – Maybe im starting a little bit above my head but I've finally been able to create my own builds of roms from source stock and cm thanks to all the info here which is pretty amazing the only time i run into problems is when i try to venture off into the custom world. My question is i guess I see so many references from devs to this and the updater script for obvious reasons i just wanted to know what it was and why their so important in the rom itself and in tweaks. Most importantly though where can i go to learn how to understand this stuff (HowTos)