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.