A series of interacting windmills placed between the two reservoirs of different pressures is presented as a model to explain general features of nonequilibrium structures. General properties of metastable states which arise when a generalized force greatly exceeds a critical value for establishment of a nonequilibrium structure are discussed from the examples of Benard convection and chemical reaction systems. Pattern formation in nonequilibrium systems is discussed with dendritic crystals and electric breakdown as examples of nonsteady nonequilibrium structures. Many of nonequilibrium structures, steady or nonsteady, seem to be explainable by a principle that the rate of increase of total entropy in a system including reservoirs is always maximum. The condition under which this principle is valid are discussed, and the principle is compared with Prigogine's general evolution criterion.