Siri for iPhone 4S is a built-in personal assistant (much like an app) that lets you speak voice commands in natural English and it brings instant answers from the web.
For instance, you may ask Siri how many calories are in a Pizza and it will bring the answer from Wolfram Alpha. Siri can help you make voice calls, reply to text messages, play songs in your iTunes library, set phone reminder, ask directions or even get weather updates by speaking the command – should I carry an umbrella?
Siri currently works on iPhone 4S only but once the beta period is over, there’s no reason for Apple to not port it to other iOS devices like the iPad and iPod Touch.