An application developer creates and tests applications designed for electronic computing devices. He or she will typically specialize in a development area such as mobile phone applications.Application developers require advanced programming skills, particularly in mobile application languages such as JavaScript, C++, Python and C#. Programming language proficiency demonstrates your ability to use the languages to build functional mobile applications.An app developer is a computer software engineer whose primary responsibilities include creating, testing and programming apps for computers, mobile phones, and tablets. These developers typically work in teams, and think of ideas and concepts either for the general public, or for a specific customer need.