Social network data consists of a collection of "nodes" (which can be any sort of entity - e.g. people, corporations) and "links" (which can be any sort of relationship - e.g. friend, sharing a board member).
Social networks is a huge and rapidly growing field which has a tremendous amount of inter-disciplinary content, with major contributions from people in a large variety of fields. One great advantage of them is their abstractness, so that insights from one field often benefit others.