A cluster refers to a subset of the nodes and edges in a graph that possess certain common characteristics or relate to each other in a particular way forming some domain-specific structure. Clustering deals with how nodes are connected and how much they are connected, that is the frequency of links between a given subset of nodes. In order to model the degree of clustering of a subset of nodes we simply take a node and look at how connected a node it links to is to other nodes that it is also connected to. For example, if this was a social network of friends we would be asking how many of your friends know your other friends, the more your friends are interconnected the more clustered the subset is said to be. This clustering within social networks is also called a clique, a clique is a group of people who interact with each other more regularly and intensely than others in the same setting.
