The POP3 (Post Office Protocol Version 3) is the network protocol used for delivery of email messages to addressees. Typically it's used in a couple with the SMTP protocol; at the same time the SMTP provides transmission of a message in path from a sender to a terminal mail server and the POP3 allows an addressee to receive this message from a server.
The IMAP is the alternative protocol for collection of messages from a mail server.