Siri is Apple's next step into the future, it's basically your Voice Command feature on crack. That is, if crack made you smart and could type for you(maybe it can 0.0). Either way this feature is pretty badass and will run your life for you. You can hold down your home button as you would with Voice Commands, except now it can do much more then pick an artist.
After activating Siri, your phone will ask you "What can I help you with?" You can respond with a numerous amount of questions. Things like "Wake me up at 6PM" will set an alarm for you to wake you up. "How do I get to the Golden Gate Bridge?" will pop up directions from your current location. Even things like "Schedule a meeting Friday at 12PM" will create an event in your calendar. As mentioned before, there are numerous amounts of possibilities.
Other then controlling your device, Siri can also type for you. Whatever you dictate to your iPhone 4S after activating, Siri will be able to understand what you are saying and type it out in a text message, application, email, and anything else.
Siri is smart, over time the speech-to-text feature will start getting used to your voice, I smell Tamagotchi in the wild. That means if your unique voice will be detected by the software and memorized. This will create an easier experience for the user and less mistakes by Siri.
This is definitely one piece of advanced technology. If you just can't wait for Siri, try out 'Dragon Go' in the App Store for free, it has a lot of the same features. Siri will be currently released exclusively for the iPhone 4S on October 14th.