In August this year Google announced to be rolling out the Knowledge Graph to countries where English is the main language and at that moment the Knowledge Graph carousel was introduced on Google.com. Google is now rolling out the feature worldwide to all English users of the search engine.
The carousel will show additional lists and collections to a search query above the search results if this is relevant. The list and collections are used when the answer to a query cannot be shown as a single entity (person or thing).
For example if you look for ´Museums New York´ Google will show the carousel with a list of different museums in New York, with the arrows left and right you can see all listings. When clicking on a museum you will perform a new search with the carousel still in place above the search results so you can easily check out all the museums mentioned.