iOS is Apple's operating system for their mobile devices (iPhone, iPad, iPod Touch). IOS can also refer to Cisco's Internetwork OS, but not typically from a user.
iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that presently powers many of the company's mobile devices, including the iPhone, iPad, and iPod touch. It is the second most popular mobile operating system globally after Android by sales. iPad tablets are also the second most popular, by sales, against Android since 2013, when Android tablet sales increased by 127%.