Not to come off the wrong way, but Clickbank's page I think does a good job of explaining what they do.
It's a website that handles sales on digital products such as eBooks. They have an affiliate program where you can select products to promote and earn a commission. The percent of commission is usually high (usually 50-75%) due to them being digital products.
If you're looking to go down the internet marketing route, you're going to have to learn to search a bit more. There's tons of useful information on this forum and around the internet.