Post Office Protocol version 3 (POP3) and Internet Message Access Protocol version 4 (IMAP4)
Both POP3 and IMAP4 are mechanisms for downloading, or pulling, email from a mail server. They are necessary because, although the mail is transported around the network via SMTP, users cannot always read it immediately so it must be stored in a central location. From this location, it must then be downloaded, which is what POP3 and IMAP4 allow you to do.
One of the problems with POP3 is that the password used to access a mailbox is transmitted across the network in clear text. That means if someone wanted to, he could determine your POP3 password with relative ease. This is an area in which IMAP4 offers an advantage over POP3. It uses a more sophisticated authentication system, which makes it harder for someone to determine a password.