Here is how it is described:
Google Now gets you just the right information at just the right time. It tells you today’s weather before you start your day, how much traffic to expect before you leave for work, when the next train will arrive as you’re standing on the platform, or your favorite team’s score while they’re playing. And the best part? All of this happens automatically. Cards appear throughout the day at the moment you need them. Appointments, commute time to work and back, flight data, sports, and a frighteningly accurate voice search functionality are just some of the “cards” you can scroll through inside of Google Now to find just what you need.
The XDA forum thread has full instructions, but be careful: you’ll have to root your Android device, modify system files, and reboot your phone a couple of times, so it’s at least annoying. Browse the forum thread before you make any changes, and be aware that in some cases it has caused apps to force-close randomly.
But finally, what are Google Now and Siri aiming at?