It’s a program that is established by a company to help it to sell products or services. It regulates all relationships between the company and affiliates who are signed up to that program. It provides affiliates a dashboard to manage advertising campaigns they run, track commissions they earn and creatives they can use to advertise the company’s products or services.
There’s a good reason why ClickBank is still a strong contender, however it does tend to focus more on digital products which may be of questionable quality. Yes, the review process is more professional these days, but it’s still primarily focused on selling digital products, especially “how to make money” courses and the like. That being said, there are some genuinely high-quality products on offer, and few affiliate programs are bigger than ClickBank, especially in selling (primarily digital) books.