The iphone SDK is the software development kit which is required for developing apps to run on IOS which is the operating system which is used on iphones and ipads. 

This software is a free download from the apple site which allows third party users to develop applications for iphones and ipads.

To get started creating apps for your iphone you will need to download this SDK.

The iPhone SDK includes the Xcode IDE, Instruments, iPhone simulator, frameworks and samples, compilers, Shark analysis tool, and more.

To download the iPhone SDK you must have an Apple ID.  An Apple ID is free and when you go to the SDK website you can set up a new account.

The SDK was originally released on 6th March 2008.  This first release was only compatible with the original iphones.  Since then ipod touches, iphone 3G, iphone 4 and ipad have been released.

With the release of this new technology the iphone IOS operating system has been upgraded a number of times to include the new functionality of the new devices.  Along with the upgrades to the iphone operating system there have been upgrades to the software development kit which allows users to develop applications.

The requirements to install the SDK is an intel based mac capable of running OS X 10.5 or above.  The requirements for OS X 10.5 Leopard:

* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor

* 512MB of memory

* DVD drive for installation

* 9GB of available disk space

If you would love to start developing iphone apps, then check out the best beginner app development book.  How to Make an IPhone App!