The long rectangular ads (728 x 90 pixel) at the top of every page and at the bottom of pages (above the quick reply) and all square ads (300 x 250 pixel) are removed. See this screen shot for an example of which ads are removed. Additionally, all ads with the 'remove this ad' link below them will be removed. Special promotions for SkyscraperPage products will remain.
What extra features come with ad-free forum browsing?