What is iPhone SDK?

The iPhone SDK (Software Development Kit) was a collection of tools, libraries, relevant documentation, code samples, and processes that allowed developers to create software applications that ran on the iPhone and iPod Touch. It included an IDE (Integrated Development Environment) called Xcode, a compiler, and other tools needed to design, develop, and debug applications using Objective-C or Swift programming languages. It's commonly used to build a wide range of apps, from games and social networking to productivity and utility tools, specifically for the iOS ecosystem.