The iOS SDK (Software Development Kit) is a collection of tools and resources provided by Apple for developers to create applications for the iOS operating system. It includes essential components like libraries, frameworks, and APIs that facilitate the development process, allowing developers to build apps for devices such as the iPhone and iPad. With the iOS SDK, developers can access features like the camera, GPS, and touch interface, enabling them to create rich and interactive user experiences. The SDK also includes a simulator for testing apps on different devices, ensuring compatibility and performance before release on the App Store.