What is the iPhone SDK?

The iPhone SDK is a software development kit that will allow third parties to create applications that can run directly on the iPhone and the iPod Touch. The kit is significant because Apple can't possibly anticipate, nor produce, all the applications that people might want to use on an iPhone. And some of those applications will convince people who weren't sure about the iPhone to buy it.

       The apple iOS SDK  (Software Development Kit) is a collection of tools and documentation that you type your code used to develop iOS apps. The main tools that make up the SDk are:
Xcode: Apple’s integrated development environment (IDE) that lets your products, type your code, trace and fix bugs, and lots more.

Interface Builder: A tool fully integrated into the Xcode IDE that lets you build your application’s user interface visually.
iOS Simulator: A software simulator simulate the functions of an iPhone or An iPad on your Mac.
Instruments: A toll that you will help you find memory leaks and optimize the performance of your apps.