Researchers in Artificial Intelligence have developed a number of different formalisms to represent and reason about knowledge.  There is a school of thought that advocates the use of logic; another school of thought has advocated the use of semantic networks.  Semantic networks are essentially graphs in which the vertices (which are typically called "nodes") represent concepts, which are connected by labeled edges, representing connnections between concepts.  Please read the wikipedia entry on semantic networks at

http://en.wikipedia.org/wiki/Semantic_network

One particular form of semantic networks, proposed by John Sowa, are conceptual graphs.  Please read the wikipedia entry at

http://en.wikipedia.org/wiki/Conceptual_graph