The skeletal system consists of 206-213 bones in adults and 300 in children. You use it when you walk (for instance).
The Muscular System:
The muscular system consists of all the muscles in your body. It is used when you start lifting weights and flexing.
The Cardiovascular System:
The cardiovascular system includes the heart and all the blood veins in your entire body.
The Respiratory System:
The respiratory system consists of the lungs, the nose, etc.
The Nervous System:
The nervous system includes all the nerves in your body and the brain.
The Digestive System:
The digestive system includes the guts, the stomach, and some other (we don't talk about this) parts. It is used for eating purposes.
The Immune System:
The immune system protects your body from microscopic invaders (bacteria)!