ASCII (ask'-ee) stands for "American Standard Code for Information Interchange." It's basically a table of numbers and their corresponding symbols. We all know that the only things computers send back and forth to each other are zeros and ones. Using ASCII as a standard, a computer can send a series of zeros and ones in a certain order and the other computer will know that it signifies a certain letter of the alphabet.
ASCII covers letters, numbers, and certain control codes. It doesn't cover graphics. Essentially, ASCII artwork denotes artwork that is created without using graphics at all. Its palette is limited to the symbols and characters that you have available to you on your keyboard. It started in the old days of computers. Most computers didn't even display graphics. So, people got creative with text and ASCII Artwork was born!
ASCII Art can be useful since many people's e-mail programs do not view graphics files without the help of another program, and there's no way to know if it does or not. Using ASCII characters to create your picture will allow you to have a picture included on the screen with your message. In order to get ASCII art to display correctly, you must display it in a font that has uniform character width. This is also known as a "fixed width font." Your browser and e-mail programs should have some provision for setting your fixed font. Courier, Monaco, or FixedSys are good bets.