Affiliate Network - is the intermediary which connects Advertiser (who own products) with Affiliates (who will promote this products)
CPA - it is the one of the most popular MODEL of work. Cost Per Action. Where you will get paid when people do some action online on Publisher source ( website, blog, social media page )
In Affiliate Networks people who promote products called Affiliates
In CPA Networks people who promote products - called Publishers
Products - Offers
This is the main difference in my opinion.