Short for Internet Message Access Protocol (Previously called Interactive Mail Access Protocol). It is used for accessing emails on a remote server while leaving them there instead of deleting them. In this way, users can access email from several different machines, once configured to use the IMAP protocol to access a certain account. (POP clients are typically configured to delete email messages from the server once they are accessed, making it possible to read one's email only on the machine first used to access it).
Unlike POP, IMAP includes functionality for remotely managing messages as well as mailboxes. IMAP has commands that allow clients to create, rename and delete folders on the mail server, as well as commands for moving messages to and from such folders. With IMAP, messages can also have meta-data associated with them. For example, IMAP servers keep track of which messages have or have not been read (such states are called "flags" in IMAP).
RFC3501 - describes the IMAP version 4 revision 1