App developers are individuals or teams responsible for creating and developing mobile applications, games or software programs. They use programming languages such as Java, Swift or Kotlin to write code that will run on a device's operating system, designing user interfaces and incorporating features based on the requirements of their clients or end-users. App developers can work independently or be part of larger development teams in companies or startups, with the goal of creating innovative applications for various platforms such as iOS, Android, Windows or web browsers.