An abbreviation for Electronic Mail, sometimes also spelt e-mail. Occasionally a ReligiousWar will erupt over the spelling.
This is mail sent and received electronically over some kind of network. The user's interface to InterNet Email is a MailClient.
Although this is by no means the only way for Email to work, these days, the mail format commonly conforms to RFC:822 and its successor RFC:2822, is MIME encoded, and is sent over the InterNet.