The levels of organization for multicellular organisms are: cells make tissues, tissues make organs, organs make organ systems, and all the organ systems combined make the organism. Thus, a body system is a series of organs that work together to complete a vital function for the survival of the organism. Your body is like a machine- if one part is not functions, then the performance of the entire organism suffers.
The body systems include: the muscular system, the skeletal system, the circulatory system, the nervous system, the lymphatic system (also known as the immune system), the respiratory system, the endocrine system, the digestive system, the urinary system (also called the excretory system), and the reproductive system.
The below links provide information on the components of each system.
The body systems help to maintain homeostasis within the body. This is done via intercellular communication, response-stimuli interactions, as well as positive and negative feedback loops.