A software development kit (SDK) is a set of tools used to develop applications for a specific operating system or platform.
These tools facilitate the development process by providing developers with "parts" they can incorporate rather than having to write the code from the ground up.
A good SDK will contain all the components a developer will need to build a new application for that particular product. SDKs may also include examples or basic test projects to help developers get up and running as rapidly as possible.
Java development kit (JDK), Windows 7 SDK, Mac OS X SDK, and iPhone SDK are examples of software development kits.