What is a search feed?
A search feed is a stream of sponsored search results that a search engine supplies to another website. The site sends a query, the engine returns matching ads, and the site displays them and earns a share of each click. Google, Microsoft Bing and Yahoo are the main sources. Access is by contract and approval, not open sign-up.
Technically a search feed is an ad request and a response. The publisher's page passes a keyword plus context such as country, device and a Channel ID; the engine runs its ad auction and returns the winning sponsored listings. The publisher never chooses which advertisers appear.
Feeds differ by engine (Google feed, bing feed, yahoo feed) and by how you hold them. A direct feed is your own contract with the engine. A hosted feed is access through a feed provider that holds the contract and takes a share. The feed is the single most valuable asset in a search arbitrage business, and it belongs to someone else.