POP3 (Post Office Protocol Version 3) is a transmission protocol used by a client to send e-mails from an e-mail server can retrieve. POP3 is an ASCII protocol, whereby the control of the data transmission is done by commands that are sent to port 110 by default.

A permanent connection to the mail server is not necessary with POP3. If necessary, the connection is created from the client to the server and then terminated again.

POP3 is very limited in its functionality and only allows the retrieval and deletion of e-mails from the e-mail server. Protocols such as IMAP must be used for further functionalities such as hierarchical mailboxes directly on the mail server, access to several mailboxes during a session, preselection of e-mails, etc.

As a counterpart to POP3 for sending e-mails, the SMTP protocol is usually implemented in clients and servers.

Source: http://en.wikipedia.org/wiki/Post_Office_Protocol