Google's Knowledge Graph is a knowledge base, launched in May 2012, which contains vast stores of structured data about people, places, events, and objects. The Knowledge Graph was described by Google as "an intelligent model - in geek-speak, a 'graph' - that understands real-world entities and their relationships to one another: things, not strings." This knowledge graph possesses pointers between objects so Google knows how the various facts stored in the knowledge base are interconnected. The Knowledge Graph is the source for the information displayed in Google's Knowledge Panel
and in other SERP features.
More information about Knowledge Graph from Google