Condensed from Wikipedia -
Electronic mail, (or email or e-mail ), is a way of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks in a store-and-forward model. Email servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously; they need connect only briefly, typically to a mail server, for as long as it takes to send or receive messages.
An Internet email message consists of three components, the message envelope, the message header, and the message body. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually descriptive information is also added, such as a subject header field and a message submission date/time stamp.
Electronic mail predates the inception of the Internet and was in fact a crucial tool in creating it.