An Apple iOS developer designs and builds applications for mobile devices running Apple's iOS operating software. They are responsible for translating technical requirements into code, developing application programming interfaces (APIs), and ensuring the performance, quality, and responsiveness of applications. They also work on bug fixing and improving application performance. Their role involves collaborating with a team to define app features, while also working on the visual aesthetics of the apps. They must have a strong understanding of iOS and its design principles, patterns, and best practices, including the core programming languages such as Swift or Objective-C.