The wiki costs money to maintain. Not very much money, but money nonetheless. We make this money back with text ads on our site. When we don't have enough money to pay for the site, a banner ad appears at the top with four text ads in it. When there is enough money, this banner disappears and there is an unobtrusive ad on the bottom left of each page.
If you really don't like the ads, there's a simple trick you can do. Visit the wiki page WikiSkins, and edit your preferences to use a skin without advertising. Ads will then stop appearing on your wiki pages.