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.
http://genealogy.about.com/od/basics/a/cluster.htm

Remember, Collateral = Direct Line Siblings and Cluster = Other Family, Friends and Associates. Genealogists who research by clusters are the most successful in extending their ancestral lines.
http://www.shoestringgenealogy.com/article/Cluster.htm