The first person to discover this pattern was Vilfredo Pareto, an economist from Italy. He concluded that 80% of all property in Italy belonged to only 20% of the population. He also suggested that this pattern could be found in many other areas and in software testing, the Pareto Principle is commonly interpreted as "80% of all bugs can be found in 20% of program modules".
Defect Clustering is based on the "Pareto Principle" and simply means that a small number of features have caused the majority of quality issues in an application. This should always be the focused area to take into account when planning future test activities.