..that Etsy uses a complex algorithm to rank search results, taking into account factors like the user's previous searches, browsing history, and purchase behavior (.pdf).
Some online shoppers mistakenly believe that using multiple keywords in a single search term improves search results.
In reality, using too many keywords can decrease the relevance of search results, according to a study published in the Journal of the Association for Information and Image Management (.pdf).
Amazon's "Frequently Bought Together" section uses collaborative filtering to recommend products that are often purchased together, leveraging the collective behavior of millions of customers.