The earlier versions of the POP protocol, POP (informally called POP1) and POP2, have been thoroughly obsoleted by POP3. In contemporary usage, the less precise term "POP" almost always means "POP3" in the context of email protocols.
POP3 and its predecessors are designed to allow users with intermittent connections such as dial-up connections to download email when connected, while the SMTP protocol was designed to allow systems with permanent connections to receive email. 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 the server requesting the data in advance.
POP3 also supports several authentication methods to provide varying levels of protection against illegitimate access to a user's email. It is also possible to encrypt POP3 traffic using SSL.
POP3 works over a TCP/IP connection using port 110.