E-mail (Electronic Mail) – sending messages from one Internet user to another.
By early accounts, the first e-mail message was sent in 1971 by an engineer named Ray Tomlinson. Prior to this, you could only send messages to users on a single machine. Tomlinson's breakthrough was the ability to send messages to other machines on the Internet, using the @ sign to designate the receiving machine.
An e-mail message has always been nothing more than a simple text message -- a piece of text sent to a recipient. In the beginning and even today, e-mail messages tend to be short pieces of text, although the ability to add attachments now makes many e-mail messages quite long. Even with attachments, however, e-mail messages continue to be mostly text messages -- we'll see why when we get to attachments.

Every person that has an account with an ISP has an E-mail address.
Every e-mail address has two parts.

weber@aol.com
The first part of your e-mail address is called your user name
The second part of your e-mail address is called the domain name

The second part of the E-mail address, the domain name, is used by Internet routers to get the e-mail to the proper ISP. Once the e-mail gets to the ISP the user name is used to get the message into the appropriate e-mail box.