What is IMAP
The Internet Message Access Protocol allows people worldwide to interact with central mail servers in order to receive, send, and generally manage their email.
The Why of IMAP
Email has come to play an indispensable part in business and recreation. That, combined with the fact that most people work on more than one machine, requires great versatility from email systems.
If emails were sent directly from computer to computer, synchronization between different machines would have been impossible. The machine where you received and opened an email would be the only place to access it from that moment on. The same goes for copies of sent messages. Imagine if, in order to find a certain email, you had to go over your home computer, your office PC, and your laptop—not to mention every Internet café you've ever visited.
IMAP is one of two main answers to that complex scenario.
The How of IMAP
Instead of storing emails directly on the machine you're working on, IMAP relies on central mail servers, which you then access with a client program like Thunderbird, Microsoft Outlook, Apple Mail, iPhone, etc.. All your email activity, whether sending or receiving, goes through these central servers.
When an IMAP client (like the list of programs mentioned above) connects to a mail server and downloads email messages, it usually downloads only a copy of these messages. If you were to connect to the same account through a different client program, from any other machine (or even the same machine), you would be able to download the same list of messages again.
The same goes for sent emails, though it may require some configuration to take advantage of this feature. Instead of firing your email directly to its destination, the IMAP client program routes it through your account on a mail server, where a copy is saved, and then sends it on to its final destination. This allows you to access all your sent emails from any machine, regardless of where they originated.
The IMAP itself is only the communication protocol—a set of commands and information formats—that allows the different client programs to speak with mail servers.
IMAP and Web-mail
The other solution to the portability problem is web-based mail, like the famous Yahoo! Mail, Hotmail, Gmail, etc.. Managing all your emails through a single website allows you similar freedom from being bound to a specific machine. However, unlike IMAP accounts, it requires you to have online access to the Internet whenever you want to go over your email, even for messages you've already read.
Today, many web-based email providers offer IMAP services: they allow client programs on your computer to connect to their server and download copies of your email records. On one hand, when you're working on your own machine, this allows you to bypass the website, or to access previously downloaded messages even when you're offline; on the other hand, when you're visiting a friend or sitting in an Internet café, you can access your account through an Internet website without the need to install and configure a local client program.