According to Wikipedia:
"Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices.

Email first entered limited use in the 1960s and by the mid-1970s had taken the form now recognized as email. Some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging.

Email operates across computer networks, which today is primarily the Internet. Today's email systems are based on 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 to connect only briefly, typically to a mail server or a webmail interface, for as long as it takes to send or receive messages."