Siri is a personal assistant created by Apple and introduced on the iPhone 4S. It allows users to control their phone and find information using natural voice commands. Siri understands spoken questions and commands and provides answers by tapping into sources like Wolfram Alpha and built-in apps. It can perform tasks like making calls, sending messages, getting directions, setting reminders and more. While useful, Siri has some limitations in terms of available languages and functions outside the US.