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 - wikipedia 
The short summary provided in the knowledge graph is often used as a spoken answer in Google Now searches.
The feature is similar in intent to answer engines such as Ask Jeeves and Wolfram Alpha and efforts such as Linked Data and DBpedia.
According to Google, the information in the Knowledge Graph is derived from many sources, including the CIA World Factbook, Freebase, and Wikipedia.
The Knowledge Vault
The Knowledge Vault is a knowledge base created by Google. As of 2014, it contained 1.6 billion facts which had been collated automatically from the Internet - wikipedia
The difference between Google's existing Knowledge Graph and their Knowledge Vault is the way that facts are accumulated. The Knowledge Graph pulls in information from trusted[according to whom?] sources like Freebase and Wikipedia, both of which are crowdsourced initiatives. The Knowledge Vault is an accumulation of facts from across the entire web. It is a mix of both high-confidence results and low-confidence or ‘dirty’ ones and machine learning is used to rank them.
History
Knowledge Graph display was added to Google's search engine in 2012, starting in the United States, having been announced on May 16, 2012 - googleblog.blogspot.co.uk