CalDAV
A protocol that extends WebDAV for accessing calendar data, used to sync between servers and clients.
CalDAV (Calendaring Extensions to WebDAV) is a protocol for synchronizing and managing calendar data over a network, standardized as RFC 4791. It extends WebDAV (Web-based Distributed Authoring and Versioning) and allows calendar create / read / update / delete (CRUD) operations over HTTP/HTTPS.
How CalDAV works
Examples of CalDAV servers
Google Calendar, Nextcloud, Radicale, and others function as CalDAV servers. On the client side, too, there are calendar apps that support CalDAV, such as Thunderbird.
Its relationship with iCalendar
CalDAV is the protocol responsible for “transferring and synchronizing” data, while the “format” of the data follows iCalendar (RFC 5545). In other words, the event data stored on a CalDAV server is in .ics format, and CalDAV provides the means to perform CRUD operations on it over the network. As the technical foundation of calendar sharing, it is used for synchronization across multiple devices.
What knowing CalDAV helps you judge
In practice, this term becomes useful when judging a switch of calendar environment or whether an integration is possible. The question “can this app sync with our calendar?” ultimately comes down to “do both sides support CalDAV (or each vendor’s proprietary API)?” With a combination of a CalDAV-compatible server and client, you can set up synchronization across vendors using a standard protocol.