POP is Post Office Protocol, a popular protocol for receiving e-mail. When using POP, messages are downloaded to the client's machine, stored there and often deleted from the server immediately. This is in contrast to IMAP where messages are always stored on the server and downloaded fresh for each session. The most common version is POP3, defined in RFC 1939. Older versions are hardly ever used.
POP3 servers usually run on TCP port 110. POP2 servers usually run on TCP port 109.