IMAP - Internet Message Access Protocol
Internet Message Access Protocol is the most widely used email retrieval protocol based on the TCP/IP protocol suite. The protocol was developed as an alternative to POP, by a person name 'Mark Crispin' in 1986. At that time POP (Post Office Protocol) was the most popular email retrieval protocol, which deletes e-mails once it downloaded to the client system, IMAP let the client view e-mail directly from a server by logging in, and doesn't deletes it until the user wants it to do.
IMAP is best suitable for today's scenario where we access our mailbox from 'array' of different devices like desktop, notebook, mobile phone or tablet. IMAP allows you to view, download, organize or even delete your emails without downloading them on a local machine. With the availability of large server capacity, great Internet speed and accessibility from array of devices, IMAP proved to be the protocol of 'today'.
Difference between IMAP and POP3
IMAP and POP3 are the most commonly used e-mail retrieval protocol. Both work on the application layer of the TCP/IP protocol suite. Like POP3, IMAP works in disconnected and offline mode too. Their similarities ends over here.
The major difference between IMAP and POP3 is that, POP3 deletes mail from the server once it gets downloaded. On the other hand, Internet Message Access Protocol keeps it on a webserver, even after the mails are synchronized with the client. In this way multiple clients can access the same mail untill any one of them manually deletes it. Due to this very reason, POP3 is only suitable for those situations where there is only a single client.