Google Now checks your email and tries to find confirmation messages for restaurant reservations, hotel bookings, flights, events you purchased a ticket for, online orders. This way, Google can show notifications and useful information without having to manually add events to Google Calendar. For example, package tracking is now a lot more convenient and you don't have to use a separate app or subscribe to email notifications.
Gmail's desktop interface shows useful links related to your messages: Google Maps for addresses, package tracking links, "add to calendar" for events. Google Now takes this to a new level: sidebar links become smart cards that trigger notifications.
Obviously, the cards are generated automatically by Google's software and no Google employee reads your email. If you don't like the new cards, you can disable them from the settings (the "Google Now" section).
Google Search for Android also brings new voice actions: you can create new events, check your schedule and launch apps.
At the moment, Google Now requires Android Jelly Bean and the Gmail integration is only available in the English interface.