The release process for Chrome is a bit different than in other browsers. While others generally follow the traditional scheme of stable releases and test releases with version numbers, Chrome uses three different channels to which you can subscribe. If you subscribe to the developer channel, you get the most bleeding edge Chrome code, with lots of updates and possibilities for breakage. The beta channel is more stable and gets updated a little less regularly, and the stable channel speaks for itself.
Chrome 3 brings several new features to the stable channel. The new tab page has been redesigned, bringing some often requested features. You can now rearrange the thumbnails by drag and drop, and you can pin thumbnails so they stay in the same place. You can customise the new tab page even further by hiding parts you don't want to see, or by opting for a simple list view of your most often viewed websites.
The omnibox has also been improved, and as a die-hard Chrome user, I can tell you that the small changes really matter here. The drop-down list now uses icons to differentiate suggestions that come from your history, your bookmarks, or from Google search results. In addition to the revamped omnibox, Chrome 3 also brings support for themes.
The other big addition is of course support for various parts of the HTML5 specification, such as the video, audio, and canvas tags. Google also throws a whole bunch of performance figures around, but those are usually meaningless: just compare Chrome 3 to other browsers yourself, and I can assure you it will render sites faster.
People who are subscribed to the stable channel will receive the update to Chrome 18.104.22.168 automatically in the background. If you want to try out Chrome 3, you can download the installer and get going.