IMAP4 (Internet Mail Access Protocol version 4) is a more advanced protocol than POP3. With IMAP4 your mail is stored and manipulated on the mail server.
So, if you move a message from one folder to the other, the message is moved on the server. If you send a message, your mail client will (usually) put a copy into your 'Sent Items' folder. If you delete a message, it is deleted from the server, and so on.
This has several advantages:
It is easy to backup your mail - you just need to backup one place, the server
You can access your mail from multiple PCs/locations. Each PC that you access your mail from will see the same message store. So, if you send a message from one PC, it will appear in the 'Sent Items' folder on all your PCs. If you read a message on one PC it will appear as 'read' on all your PCs, etc
You can share message folders. With IMAP4 several people can access a mailbox/folder at the same time (unlike with POP3). VPOP3 supports IMAP4 ACLs (Access Control Lists) so you can tell VPOP3 that some of your folders are accessible by specific other people, and that they can only read messages, not delete them, etc.