From a stickie on my forum:
Resistance training is simply the act of exerting force by way of muscular contraction against a given load. Resistance training is often called strength training or weight training as well. This load can come externally using equipment such as barbells, dumbbells, kettlebells, bands, medicine balls, etc. Internal resistance is applied using your own bodyweight. When applied appropriately, resistance training generates positive adaptations/responses in the body. The adaptations we’re most interested in are primarily associated with muscle and neural qualities. These qualities include but are not limited to increased strength, muscular endurance and muscle mass.