http://genealogy.about.com/od/basics/a/cluster.htm?nl=1

Our ancestors did not live in isolation, although we often research
them as if they did. They were part of a family, with siblings,
parents, aunts and uncles, cousins and other relatives. They were also
part of a community, with friends, neighbors, classmates, and
co-workers. This "cluster" of family, friends and neighbors can
provide valuable clues to the lives of our ancestors.

Cluster genealogy, sometimes referred to as whole family or extended
family genealogy, is the practice of extending your research on a
person to the individuals and families to which he was connected.
These connections could range from his brother or spouse, to the
neighbor who appeared as a witness on a land deed.

Why Cluster Genealogy?

Read more at

http://genealogy.about.com/od/basics/a/cluster.htm?nl=1