This RFC outlines the format for iCalendar data and registers a MIME type "text/calendar" for it.
Since iCalendar data is the common format which should be understood by calendaring software, it is important to understand it before developing data structures or functionality for an application.
iTIP describes methods for exchanging iCalendar data to perform common tasks such as scheduling, rescheduling, or cancelling meetings. It adds semantics to the syntax described in RFC2445.
This RFC extends RFC2446 and applies it specifically to the problem of exchanging iCalendar data via email.
CAP is a protocol for CUAs (Calendar User Agents -- the calendar software you have on your desktop computer, handheld, etc) to talk to calendar servers. The protocol is currently being drafted.
This is a draft by the IETF working group which gives an overview of the main concepts covered in the iCalendar RFCs. It's slightly more technical than this document you're reading.