Mac users who use Google Calendar and iCal to manage their on and offline calendaring have had to use a myriad of third-party products to keep the two in sync. That's changed now that Google is including CalDAV support as part of Google Calendar's built-in functionality. This means you'll be able to make changes in iCal and have them instantly reflected in Google and vice-versa. Previously that data swap was a one-way affair, with users simply subscribing to their Google Calendar feeds in iCal.
CalDAV is a standardized sharing protocol, allowing you to access the same set of data with others to edit and sync data changes between multiple users. On a workflow level, this means people with access to edit your Google Calendar will be able to make changes in both iCal and Google Calendar that will sync up with both.
To make use of it on a Mac you'll need the latest version of iCal, which comes with OS X (Leopard). It has native support for CalDAV subscriptions, as does Mozilla Sunbird, but Google says it's currently not supported.
One thing to note is that the current iteration of CalDAV support on Google's part is not perfect. The support article on known issues is somewhat lengthy, with show-stoppers like the inability to create new iCal calendars from Google Calendar, and reminders and alarms not moving to the other platform when created. Until some of those issues are ironed out, getting perfect sync will require one of the many third-party tools that does the legwork for you (see Calgoo, Spanning Sync, and Plaxo).
[via Google Operating System]