"Social capital is the network of relationships between individuals, groups, and entities. This term is used in several different ways in fields like economics, sociology, and anthropology. In all cases, the use of this concept boils down to the idea that people create connections with each other, and those connections are used in a variety of ways. Studying social capital can be a way to learn about how a society functions.
In a classic example of social capital, people who attend the same university tend to feel connected with each other, both because they may have interacted on the campus and created friendships, and because they have an institution in common with each other. Two graduates of the same university are more likely to connect with each other because they share social capital, and this can translate into advantages for one or both parties."
(Copied from What is Social Capital on the wiseGEEK website.)