It was designed to work with intermittent connections such as dial-up connections, whilst the earlier SMTP protocol was designed to work with permanent connections. POP3 is a pull protocol (where the client requests data from the server), whereas SMTP is a push protocol where the client sends data to the server without being requested to.
An important difference between SMTP and POP3 is that SMTP normally does not include any user identification through a login-id and password. This makes SMTP often unsuitable to downloading messages to an individual recipient.
POP3 works over a TCP/IP connection using port 110.
Other mail protocols include SMTP and IMAP.
RFC 1939 - describes the POP3 - Post Office Protocol version 3.
http://junior.apk.net/~qc/comp/protocols/pop.html
wikipedia.org dumped 2003-03-17 with terodump