Dev:Foundation.framework

This is the basic Objective-C-based framework, used by everything. Provides NSString, NSDictionary, NSArray, ... It provides many classes that wrap classes, but also provides unique functionality above and beyond that.

Mac OS X classes that are undocumented on the iPhoneOS
There are several classes that are present on both Mac OS X and iPhoneOS, but the corresponding documentation are not found on the iPhoneOS.

Environment variables
Application built with Foundation will recognize the following environment variables