It's a standard format so you can check the RFC specifications;
http://www.ietf.org/rfc/rfc2445.txt

Might try looking at one of these two links

http://support.microsoft.com/kb/287625 (1.0 format - vcs)
http://support.microsoft.com/kb/307313 (2.0 format - ics)

Other than that, what I usually do learn about this format is to create a
calendar item and then save it to the file system.  From there, open in
notepad to get a better idea of what properties are being set.  Once armed
with that, much easier to Google each property to see what the accepted
values are since it should relate back to RFC 2445/2446.