Ad units: These refer to the different types of ads which appear online, including banners, interstitials, pop-ups, skyscrapers and text links.
It is generally considered optimal to place the ad unit near the top of the page. Ideally the ad unit will have some type of shading that stands out from the rest of the web page, but not in an obnoxious way. The background and text should be different enough that the text is clearly visible. Using colors that are too similar makes the text difficult to read.