POP3 (short for Post Office Protocol, version 3) is one of the two most widely used Internet protocols for getting e-mail. The other is IMAP (Internet Message Access Protocol). Modern e-mail clients have no problem supporting both of these protocols, and virtually all of them support both protocols.
POP3 is the application layer of the Internet Protocol that allows email clients to access a remote email server over a TCP/IP connection. Similar to IMAP, POP3 is supported by the most well-known web-based email protocols, such as Gmail (Google Mail), Microsoft's Hotmail, and Yahoo! Mail.
POP3 supports simple access and delete requests. Although most POP3 clients support features such as leaving the email on the server after it has been downloaded, clients typically just connect, download any email, save it on the user's computer as new email, delete it from the server, and disconnect.