IMAP (short for Internet Message Access Protocol) is one of the two most widely used Internet protocols for getting email. The other is POP3 (Post Office Protocol). Today's email clients have no problem supporting both of these protocols, and virtually all of them support both protocols.
IMAP is the application layer of the Internet Protocol that allows email clients to access a remote email server. IMAP can operate in both online and offline mode. An email client using IMAP will usually leave emails on the server unless the user deletes them. Because of this and other specifics of IMAP, multiple clients can use it to manage the same e-mail box.
IMAP together with POP3 is supported by the vast majority of email clients, but fewer email services support IMAP.
Advantages of IMAP over POP3:
The disadvantage of IMAP is the high complexity of the whole protocol and complicated implementation.