POP3
POP3 is an Internet protocol for retrieving e-mail.
When using POP3 to check e-mail, an e-mail client connects to the POP3 server and downloads e-mail messages to your local machine. In most cases, the client then deletes the messages from the server.
Compared to IMAP
IMAP is a newer, more capable protocol than POP3. POP3's simplicity makes developing e-mail clients for it easier, however this also means that several features found in IMAP are unavailable in POP3.
Because POP3 downloads and then deletes message from the server, it should only be used in situations where the user only wants his or her e-mail to be on a single computer (and one on which the user will do regular data backups). IMAP, on the other hand, leaves e-mail messages securely on the server, so it is useful in situations where the user might want to check his or her e-mail from more than one location.
People with Gustavus accounts are strongly encouraged to use IMAP over POP3.