IMAP is Internet Message Access Protocol, a common protocol for receiving e-mail. When using IMAP, messages are stored on the server and downloaded to the client's machine only temporarily for viewing. The alternative to IMAP is POP, where messages are downloaded permanently. IMAP 4 is defined in RFC 3501; IMAP 2 is defined in RFC 1176.
IMAP 4 servers usually run on TCP port 143.