Don't know what they do? See this from their website
A new way to earn money from your website or blog, by selling books, DVDs, and CDs. You'll earn 50% of the proceeds above the cost of products sold.
You decide exactly what kind of books and music you want sold on your site. You even decide what colors to use in the product window to match and feel like a part of your website.
You only offer products you want, in the way you want it to look, and we take care of the rest
When you copy their codes there aren't choices for the width and the height but you could adjust them to suit. I have changed them from 160x600 to 200x650 just to show all words in the ads.
Worth also seeing TechCrunch's coverage here.