The actual graphical advertisement the user is exposed to. Common formats are GIF, JPEG and HTML5 (earlier Flash), and also MOV, FLV, MP4 (video).
A creative may include sound, video, animation, and/or traditional text.
The size and form, both of which are regulated by Internet Advertising Bureau (IAB) standards, as well as the type of action it promotes (clicking, downloading, filling in information, etc.) influence the cost and the success of a campaign.