This definition is from the ComputerUser.com High-Tech Dictionary:
Internet Message Access Protocol. A protocol that allows a user to perform certain electronic mail functions on a remote server rather than on a local computer. Through IMAP the user can create, delete, or rename mailboxes; get new messages; delete messages; and perform search functions on mail. A separate protocol is required for sending mail. Also called Internet Mail Access Protocol.
So it can be considered a data storage. To use it you you need an IMAP server, such as Cyrus, Courier or UW.