iPhone SDK - The iPhone SDK, or Software Development Kit, is a software development environment that allows developers to write programs that can be installed and run natively on the iPhone, iPod touch, and iPad, AKA apps.
When it was first announced, the iPhone allowed only web-based applications, but not apps that users could install on the devices themselves. The drive to create native apps was strong, though, and developers soon enough cracked the iPhone's security and began creating their own apps, with various responses from Apple.
Since the release of the SDK, the App Store environment has become a major differentiator and revenue source for Apple. As of May 2011, the App Store has nearly 500,000 apps in it, the vast majority written using the iPhone SDK.
The SDK includes a development environment, desktop iOS simulator for testing, optimization tools, and drag-and-drop interface builder, and access to documentation and support.
Getting the SDK
In order to get the SDK, developers must register with Apple's iOS Developer Program. This includes creating an account with Apple and paying a US$99/year fee. When that it done, developers can download the iPhone SDK from Apple's website.
A Mac with an Intel processor and Mac OS X 10.6 or higher. Computers running Windows are not a supported development environment for the iPhone SDK.
Apple announced the iPhone SDK in October 2007. The SDK was released in March 2008.