It attempts to act somewhat as an automated "personal assistant".
More documentation: https://support.google.com/websearch/answer/2819496
What type of cards does it show?
Updates within Google now are shown as cards:
- Boarding Pass - retrieved from Gmail
- Flight Information - from flights from boarding passes
- Package Tracking - retrieved from Gmail
- Sporting events (and scores) - manually added from the settings configuration
- Appointments - retrieved from Google Calendar
- Travel to your next destination - from airline confirmations emails in Gmail
- Weather - from Geolocation API on the device
This information is gleaned from any number of sources, from your Inbox, Calendar and also from web sites you visit frequently.