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Frameworks are collections of code written by someone else you can easily include in your project.
There are two types: Cocoa frameworks (iOS frameworks) and 3rd-party frameworks (aka vendor frameworks). Anyone can compile their iOS code into a framework and expose it to other developers without passing around their source code.
The act of including a framework into your project is known as linking to it (alternatively, linking against).
The steps are as follows, assuming you are trying to link to the MapKit framework: