What is the difference between POP and IMAP?
POP is the Post Office Protocol and is designed to (typically) deliver the entire contents of your messages (including attachments) to whatever device you are reading your messages from. Once the mail is delivered and you disconnect, the mail is removed from the mail server and is no longer accessible by talking to the mail server.
IMAP is the Internet Mail Access Protocol. It was designed to overcome some problems with POP behavior and provide more features for delivery and management of e-mail. With IMAP, mail is kept on the mail server and is managed there by a series of commands sent to the server by your client. Copies of messages and attachments are transferred to your client only when you request them. By default, only descriptive information about your messages is sent to your client. This works very well over slow links or for access from devices with limited computing or storage capacity such as a PDA (ex. Palm Pilot) or web browser.
The advantages of IMAP is that it is more feature-rich and allows you to read your e-mail from any location and any device with IMAP support.
The advantage of POP is that it has been in existence longer and therefore has better support in most mail clients. Also, since POP mailboxes are local, you have direct control over the physical location of your e-mail messages.