what file type is a webinar?

Dear Evaphilpot, 

Unlike what the commentator said, a webinar is not a group of
softwares, but a concept describing web activity. Those softwares
mentioned would help you do a webinar.

A webinar is a hybrid word, composed of "web" and "seminar". "Short
for Web-based seminar, a presentation, lecture, workshop or seminar
that is transmitted over the Web. A key feature of a Webinar is its
interactive elements -- the ability to give, receive and discuss
information. Contrast with Webcast, in which the data transmission is
one way and does not allow interaction between the presenter and the
audience."
(SOURCE: "What is a Webinar", Webopedia,
<http://www.webopedia.com/TERM/W/Webinar.html>).

More information: 
Wikipedia, Webinar
<http://en.wikipedia.org/wiki/Webinar> 

I hope this answers your question. Please contact me if you need any
clarification on this answer. My search strategy was:
1. To search Google using the rubric [define:webinar], which brings
definitions, such as Wikipedia;
2. I could also search using the exact term "what is a webinar", which
would have brought pages - such as the Webopedia - discussing and
defining it.

Webinars is more of a "group" of software then a particular file
format. Webex, Macromedia, Webconference.com and others all have some
sort of screen capture. Some of them are proprietary, some aren't.
From experience of using all three, they all let you save in either
AVI or MPEG. But it differs per company.

If your looking at capturing a webinar for future reference, I would
suggest using Camtasia found at www.techsmith.com. I used to record
Webex and Placeware conferences all the time for future reference.