General question about POP, IMAP, Webmail

Hi there

I've done a lot of reading on Google, but they all seem to be very simple definitions.

I understand the basic differences between POP3 and IMAP4.
However, my doubts lie within webmail.

Is webmail just a shell? (as in like a regular email client like Outlook, but just web based, i.e. using http(s) ?)

Or, is it a protocol?

How would I tell if an existing webmail client has both the ability to support POP and IMAP? (or if it's using one or the other?)

I'm thinking that this is a very basic question, but it definitely would help to see an answer to understand the more complicated issues revolving around email

Cheers
Imwired