Well, I just got Xcode 4, and at some point it starts to load something called the Mac OSX 10.6 core library. What is this and why?
This is Apple's documentation for Mac OS X, all class references, etc. With it, you can search for various information about each class that Apple offers you (for example, if you want to learn more about methods in NSString).
You can manually download and extract all the Xcode 4 documentation by getting the links and instructions here: http://learning2code.blogspot.com/2008/05/download-xcode-docsets-to-your-hard.html