The abbreviation IMAP stands for Internet Message Access Protocol. The IMAP protocol allows access to and management of received e-mails. In contrast to the POP3 protocol, the mails usually remain on the mail server and are only transferred to the client computer when required.

IMAP was designed with the goal of providing access to mailboxes and messages as if these would be on the local computer. 

Advantages:

 - Access from several computers (e.g. from work, at home, on the road)

- Central storage and archiving of e-mails

- Shared mailboxes for multiple users possible

- Some server implementations provide permissions for mailboxes (read, write, delete, etc.) per user,

  administration etc.) per user and thus allow a detailed access control.

- Searching and sorting can be performed on the server side, so that even clients with low

  CPU power can efficiently use large mailboxes

- Reduction of the local data volume, especially important for mobile phones and the like.

Disadvantages:

- Some programs cannot access e-mails without an existing Internet connection: 

- some programs cannot access e-mails without an existing Internet connection, because they can't

  do not support the "disconnected mode" of IMAP

- increased demand on the performance of the server compared to POP3: IMAP is in contrast 

  to POP is a session-oriented protocol, some with very long TCP connections.

Source: http://en.wikipedia.org/wiki/IMAP