I run the Affiliate Tier of the Developers Program at eBay.
As you probably know, eBay started as an auction site and is now the largest e-commerce site in the world. A Developers Program creates tools for programmers so their software can automatically interact with eBay.
Affiliates are people who have a website with special links to other websites. When people follow these links and buy something on the target website, the affiliate gets a commission from the owner of the target website.
Not all sites offer affiliate commissions. eBay does, through the eBay Affiliate Program. eBay provides:
text links
Click here for eBay!
buttons

and
widgets
which affiliates can include on their website.
My program (the Affiliate Tier) is for software developers who want to create more sophisticated websites. These “Affiliate Developers” sign up with me and get access to the eBay Application Programming Interface (API), also called eBay Web Services.
With our API, they can present anything that’s currently on eBay on their own website to entice people to go look at items on eBay. For instance, they can show all U2 CDs available right now on eBay. They can show statistics about how many items are on eBay, category lists to help you find what you want, ... in short, anything that would improve a users' shopping experience.
A great example is Verizon's Superpages, which shows you eBay items related to your online yellowpages search!
I've just surpassed the milestone of 1,000 affiliate developers in my program. If you'd like to learn more, feel free to check out developer.ebay.com/affiliates.