As organisms live and grow, they're constantly in the process of making and breaking bonds in molecules. Metabolism is the sum of all the chemical reactions that take place in an organism. Metabolism includes catabolism and anabolism.
Catabolism includes the processes that break down complex molecules into simpler molecules while harvesting their energy and storing it, usually in the form of ATP. Anabolism includes the processes that build more complex molecules from simpler molecules. The energy acquired through catabolic processes is used to drive anabolic processes. Neither catabolism nor anabolism is completely efficient, so at each step, some of the available energy is lost into the environment as heat.