POP (Post Office Protocol)
A POP email client downloads all of your e-mail from the e-mail server to your own computer, then deletes it from the server. Once you have checked your messages via POP, those messages no longer live on the server – they exist only on the computer from which you checked your e-mail.
POP is really only sufficient if you have one dedicated computer from which you check e-mail.
IMAP (Internet Message Access Protocol)
An IMAP client synchronises the e-mail on your computer with the contents of your account on the e-mail server. IMAP copies messages from the server instead of deleting them; when you use IMAP, an e-mail message is only removed from the e-mail server when you choose to delete it. Instead of moving messages from the server to your computer, IMAP synchronizes your computer with the e-mail server.
IMAP is a good choice for people who need access to their e-mail from home, the office, the road, and anywhere else.