See my WDVL article, They're lookin' but they ain't buyin' for tips on how to design your site to sell. In fact, low conversion rates are one of the most commonly-heard complaints about marketing on the Web. On the face of it, this would seem to have nothing to do with banner ads. After all, a banner can lead a horse to your site, but if he won't drink once he's there, the fault would seem to lie with the site designers. In fact, however, a banner ad campaign often has an indirect influence on a site's conversion rate.
Banner ads attract prospective customers, but some prospects are hotter than others.
A given banner on a given site may attract a high proportion of people who are seriously interested in a product, while another one attracts mostly window-shoppers. Both the banner and the site on which the banner appears may influence this. Banners that provide facts about the product being advertised tend to attract more likely buyers. As for the site the banner appears on, it's generally believed that the better the targeting, the higher the quality of the clickthroughs. While this all sounds very logical, in the real world it's almost impossible to measure conversion rate for individual banners, at least in a large campaign.