Electronic mail, abbreviated e-mail or email, is a method of composing, sending, and receiving messages over the Internet.
It is free (other than any cost of using the Internet) and you can usually easily send attachments such as photos and spreadsheets. On the down side, it can be hard to find someone's e-mail address; if you get it wrong and send a message to the wrong person, you may never know if the incorrect recipient just ignores it. Also, unless you know that the recipient checks his or her e-mail regularly, you don't know how long it will be before your message is read. It is sometimes, but not always, possible to get a receipt - a message that the recipient has opened the e-mail.
For a more detailed description and history read the Wikipedia article on E-mail.