Siri is described by Apple as the "intelligent personal assistant", which means that you can make verbal requests to Siri and she does an amazing job of complying with the request. Presently Siri is available on the iPhone 4S and 5, iPad 3, as well as the 5th Generation iPod touch and requires Internet access. To use Siri just hold down the Home button (make sure that Siri is on in Settings - General - Siri), and the microphone will light up as she detects your voice. The more you use Siri the better she becomes at understanding your voice, in addition she uses information from your contacts, calendars and reminders to better decipher your speech requests.
There are numerous things you can ask Siri to do, for example, "Call my sister Sherry" or "Give me directions home" or "What is Apple's stock price?" If you are interested in learning more about about Siri's vast repertoire of commands simply tap the i located to the right of the question mark when Siri is activated to access the guide or visit a great article on the Tech Blog for a full list of commands.
Also if your car is equipped with Bluetooth, Siri can help you be a safe driver. Activate Siri via your car's Bluetooth by pressing and holding the call button.
Oh, and did I mention that Siri speaks multiple languages!
As a side note, given that the discussion is about talking to your idevice, the iPhone 4S and newer, iPad 3 and newer, as well as the 5th Generation iPod also provide speech to text whenever you use the keyboard. A microphone is located to the left of the keyboard's space bar and for students who struggle with written output or spelling this built-in assistive feature can be very helpful!