.h

A program header file is used to separate the interface from the implementation. It defines what can be done but not how it is done.

.m

A program implementation file. It defines how things are done..

.xib

An Interface Builder file. It defines what the program interface, including windows, buttons, text fields, and so on will look like.

.plist

A property list. It contains a list of keys and corresponding values. The Info.plist includes important App information (like App name, App version, App copyright, and so on.)

.strings

A list of strings that are localizable ( translated into other languages). The InfoPlist.strings file contains localized versions of some of the strings found in the Info.plist file.

.pch

A precompiled header file. When we build our program the files are compiled into a single runnable file. Because compiling takes time, the compiler will precompile the .h files and save them in the .pch file. This is used to make building the App faster.

.rtf

A rich text formatted file. Rich text, differs from standard text, in that it can contain things like bold, italics, and embedded images. The Credits.rtf file is automatically displayed by the standard About window for our App.

.framework

A framework is a bundle (and a bundle is just a folder that looks like a file) that contains .h, .xib, and library files. Frameworks are provided by Apple to perform common functions (like displaying windows) so that we don't need to write that program code ourselves.

.app

An app is a bundle that contains everything our App needs to run.