Flipping websites is the act of buying, improving and then selling on a profitable web business. It is different to buying and selling domains in that the valuation is based on the monthly net profit and all other assets (domain name, email list, social profiles) are already factored into the price.
A content site (often known as an authority site) builds an audience over time. It is about returning visitors as much as new visitors, it publishes new content regularly and looks to convert visitors into subscribers. It uses a variety of monetization channels that includes affiliate marketing as well as advertising and digital products. The website is the business and only asset, unlike ecommerce where there are products / inventory, and unlike SaaS where there are software applications / development.
An affiliate site (often known as a niche site) focuses on affiliate marketing as the monetization channel, it typically only caters to new search traffic every month and content is not regularly added or updated.