Put simply, Google says Buzz is a new way to start conversations about things you find interesting. It allows you share status updates, photos, videos, RSS information, and more with existing Gmail contacts, all within Gmail's infrastructure. Naturally, relevant Google services such as Picasa are implemented, but Buzz also connects to sites like Flickr and Twitter.
Buzz also has mobile-specific features that rely on position location. For instance, you can tag your location when adding a new post from your handset. The mobile version can also show you nearby activity of other Buzz users, whether you're following them or not. A feature called "Buzz Layer" reveals a (presumably) user-tagged map and shows you what's happening in a given area.
The service has officially launched but the complete roll out will take a few days. Google says it intends to make Buzz a fully open and distributable platform. The company is working on a suite of open protocols to create a complete read/write developer API, and developers interested in participating should head over to Google Code.