POP3 is an acronym of Post Office Protocol 3. POP3 is a protocol that provides a simple, standardized way for users to access mailboxes and download messages to their computers. What it does, is,it downloads mail from your mail-server and stores it for the user, till the user downloads the mail.
The stored POP3 mails get deleted as soon as the user downloads the stored mails. It can be be thought of as a ‘store-and forward’ email service. Thus this allows users with temporary internet connections to view mails when online, and to view and manipulate messages these stored messages when user is not connected.
IMAP (Internet Message Access Protocol) is the other most prevalent Internet standard protocols for e-mail retrieval.