Clickbank sells only digital products, unlike CJ, which also has physical stuff. Plus there are no application forms to register with Clickbank. It's just 5 minutes work.
CJ deals with larger companies and household brands mostly so the products are more varied and typically of higher percieved value. Payouts are smaller but the marketing assets are ready made for youl. CB is for small shops working on a shoestring selling ebooks, software and other downloadable digital products. Payouts are a little higher since digital products have high margins. Most of the products don't offer marketing collateral such as banners, email ads etc.