CalDAV, for the non-programmers among you, is an open extension to the Web Distributed Authoring and Versioning (WebDAV) standard for Web-based calendar services. It's used in Apple's iCal; Mozilla's Calendar Project-based programs; and it will soon be available in the next update to Windows Phone 8. It's as close to a lingua franca for calendaring applications as we have today.
Its close relative CardDAV is an address book client/server protocol designed to allow users to access and share contact data on servers. It's also widely supported in mobile operating systems—Android, iOS, and Blackberry QNX—and numerous e-mail programs.
Piotr Stanczyk, the Google Calendar APIs group tech lead, also announced that both APIs will be integrated with the Google APIs Console. In addition, developers will be able to use the OAuth 2.0 protocol for user authentication and authorization. Indeed, developers must use OAuth 2.0 to access Google's take on CalDAV and CardDAV.
What all this means is that developers will be able to create applications that can integrate Google calendars and contacts. For users, this means you'll continue to have a broad choice of apps that can use Google Contacts and Calendar for your data. The one major exception remains Windows 8 and such Microsoft programs as Outlook.
In the past, Microsoft had relied on Google's support for Exchange ActiveSync (EAS) for for syncing e-mail, contacts and calendars, but Google dropped EAS support for free e-mail clients in December 2012. While Microsoft will besupporting CalDAV and CardDAV in the next Windows Phone 8 update, the boys from Redmond still haven't said if they'll add support for either protocol in Windows 8 or its successor, Windows 8.1. I rather hope they do.