Webmail is a way of accessing your mail using a normal web browser. It is designed to be close in functionality to a 'normal' email client, so should be straightforward to use.
If you use VPOP3 Enterprise, then the mail visible through the web browser will be the same as that visible through your IMAP4 email client - the IMAP4 client will synchronise with Webmail.
If you use VPOP3 Basic, then you will usually only be able to see new mail in the 'Inbox', and if you move messages to other folders in Webmail, they will not be visible in your email client, and vice-versa.