A piece of advertising. At first it was called advertisement, afterwards advert, and now just ad. They are very common in the internet. They can be displayed as popups, popunders, banners, skyscrapers, text ads and in many other ways.
Some companies exploit them as adware - programs that advertise on the computer they are installed on. Normally the user doesn't know that they're installed until seeing the first ads. Although, most people avoid using such over-advertised products because they have no interest in supporting such companies that exploit adware.
Some web browsers, like Firefox, have the ability to have add-ons installed. Such add-ons include ad blockers like Adblock Plus, that help the user remove unwanted flash, text, picture, and whole frame ads. For other browsers, users can install additional software that remove ads.
John: "Help me! I can't get rid of all those ads!"
Jake: "Don't worry! I know some good ad blockers."
Webmaster: "Man, since I use that adware you wrote my site has lost visitors!"
H4cK3R: "1 ' ]) l3 3 7 7 3 ® ® 3 /\/\ 0 \/\/ 3 17 !"