iOS applications are software programs designed to run on Apple's mobile operating system, iOS. These apps can be downloaded from the App Store and are available for devices like the iPhone and iPad. They serve various purposes, including games, productivity tools, social media, and more. Developers create iOS applications using programming languages such as Swift or Objective-C. Apple provides a set of guidelines and tools, including the Xcode development environment, to help developers build and optimize their apps for performance and user experience.