System Archetypes are patterns of behavior of a system. Systems expressed by circles of causality have therefore similar structure. Identifying a system archetype and finding the leverage enables efficient changes in a system.
More info here: http://wwwu.uni-klu.ac.at/gossimit/pap/sd/wb_sysarch.pdf
Ten archetypes are generally acknowledged as forming the set of tools that reveal patterns of behavior in systems.