POP - Post Office Protocol

Post Office Protocol is one of the most widely used application layer protocol used to retrieve e-mails from a remote server. POP was primarily released in the year 1984, its revised version, POP2 was released in 1985. Its current version POP3 was developed in 1988. 

Working of POP
This TCP/IP based protocol, when connected to the mail server, retrieves emails and download it to the client system or computer. Once they are downloaded, they will be erased from the remote server. That means, if you try to access those mails from any other device, it won't work.

The protocol is suitable for those who access their mailbox only from a single device. If you having multiple clients checking a mailbox, a better solution is to have another e-mail retrieval protocol, IMAP (Internet Message Access Protocol). IMAP, like POP, reads mail from a mail server, but unlike POP doesn't deletes them after reading.

Both, POP or IMAP, needs another application layer protocol - SMTP (Simple Mail Transfer Protocol) - to complete its mail retrieval process. SMTP is basically used to send email across the Internet. The mail handler receives the mail, and that mail is read either through IMAP or POP3.

Today, all modern e-mail clients like outlook & Mail (Mac OS X), and webmail service providers such as Yahoomail, gmail and Outlook.com supports POP3 and IMAP.