These are different Internet protocols, but they are all basically designed to provide the same thing -- access to e-mail.
Post Office Protocol 3 (POP3) is one of the early Internet standards for messaging. Clients can access their inboxes, but that's it; no other folders on the server can be accessed. POP3 only handles retrieval of messages from a client's inbox. For more information on POP3, refer to RFC 1725, RFC 1734 and RFC 1939.
Internet Mail Access Protocol v4 (IMAP4) is another early Internet standard similar to POP3 but more full featured. IMAP4 can be used to access multiple mailboxes and mailbox folders, and public folders, as well. For more information on IMAP4, see RFC 2060 and RFC 2061.
Webmail (a.k.a. Outlook Web Access) uses HTTP/HTTPS to provide Web browser-based access to clients' inboxes, their other mail folders and to public folders.