POP, or Post Office Protocol, is an email protocol used to retrieve messages stored on an e-mail server. With POP access, you can download and access your other emails using Microsoft Outlook Express , Netscape Mail , or Mozilla Thunderbird etc., even when you are not connected to the Internet. POP basically allows you to retrieve, and manage one mail account from another. Post Office Protocol version 3 (POP3) is the most common protocol for receiving email messages.
You can receive Gmail, Hotmail and Yahoo mail with your mail client with POP3 access.
POP and IMAP (Internet Message Access Protocol) are the two most important Internet standard protocols for e-mail retrieval, All modern e-mail clients and servers support both POP and IMAP.
POP vs IMAP
Unlike POP, IMAP keeps your messages on the server. When you click to read a message IMAP presents the message that is stored on the server. With POP, Messages are stored on your computer(Outlook Express etc). If your computer fails you may lose all your e-mail.