An introduction to recommendations and popular items in SharePoint Server 2013
In SharePoint Server 2013, recommendations and popular items can be displayed because a feature called Usage analytics tracks and analyzes how visitors interact with your website. You can use the results that Usage analytics produces to add content to your website, for example “People who viewed this item also viewed” or “Popular items in this category.”
Here’s how Usage analytics works:
Visitors do something on your website, for example, they view an item. This generates a usage event.
The usage event is recorded in the Event store.
The usage events are sent to the Analytics Processing Component where they are analyzed. The result is sent to the Search index.
When visitors navigate to a page that contains a Recommendations or Popular Items Web Part, a query is automatically issued and sent to the search index.
The query results are returned from the search index and displayed in the Recommendations and Popular Items Web Part on your website.
Further Reading:
http://blogs.technet.com/b/tothesharepoint/archive/2014/01/16/an-introduction-to-recommendations-and-popular-items-in-sharepoint-server-2013.aspx