Which iOS frameworks most useful to learn and in what order?

Question

I've just started learning Obj-C and the Cocoa-Touch framework for developing iPhone and iPad applications. When you add a framework to an XCode project I can't help notice there are an enormous number to get familiar e.g. AddressBook, CoreData, Foundation, Security etc.

• Which frameworks are common to most projects and the most important to master i.e. UIKit?
• Which frameworks will require a prerequisite level of understanding of another?
• Which frameworks are application specific and can be left until needed i.e. Accelerate?