From ACal Wiki
CalDAV is a protocol for sharing Calendar data among multiple client programs. The protocol itself is based on the "Distributed Authoring and Versioning" standard commonly known as "DAV" or "WebDAV" and it uses the HTTP protocol for transport of the data and metadata about the calendar.
CalDAV extends from some earlier standards, such as GroupDAV, providing capabilities which are specific to calendaring, and assisting client software to minimise the amount of data communication between the regular synchronisation of client and server.
The CalDAV standards are open standards, handled through the IETF standardisation process as "RFC"s.
For a list of the CalDAV servers supported by aCal see the Compatibility page.