The Knowledge Graph is a knowledge base used by Google to enhance its search engine's search results with semantic-search information gathered from a wide variety of sources. Knowledge Graph display was added to Google's search engine in 2012, starting in the United States, having been announced on May 16, 2012. It provides structured and detailed information about the topic in addition to a list of links to other sites. The goal is that users would be able to use this information to resolve their query without having to navigate to other sites and assemble the information themselves.
According to Google, the information in the Knowledge Graph is derived from many sources, including the CIA World Factbook, Freebase, and Wikipedia. The feature is similar in intent to answer engines such as Ask Jeeves and Wolfram Alpha and efforts such as Linked Data and DBpedia.
As of 2012[update], its semantic network contained over 570 million objects and more than 18 billion facts about and relationships between different objects that are used to understand the meaning of the keywords entered for the search. 
In August of 2014, Google announced a new initiative, the Knowledge Vault, which derives much of its data from the Knowledge Graph and the sources thereof, as well as harvesting its own data, ranking its reliability and compiling all results into a database of over 1.6 billion facts collected by machine learning algorithms.
We have had internal debates about when to ship something. We could come out with something now like them, but it wouldn't be state of the art. It's too constrained to be an agent now. We are not shipping until we have something more revolutionary than evolutionary.
During the Google I/O conference in May 2013, Google's Amit Singhal presented on the future of search, explaining that a search engine's three primary functions will need to evolve and that search will need to: 1. Answer, 2. Converse, and 3. Anticipate. As part of his keynote talk, Singhal stated, "A computer you can talk to? And it will answer everything you ask it?"
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- Official website
- Why Google Carousel is Important to Local Business at Web-matic 24/7. Matthew Johnson (with audio)
- How the Knowledge Graph Affects Organic SEO at Return on Now.