It's easy to point out a few in your body. The two you think of the most are probably your respiratory and digestive system. A couple of times a day you might get hungry, sit down, and have a nice meal. All of that food gets broken down in your digestive system so that your body has energy to survive. The stuff your body doesn't need is pooped out at the other end.

Since you're breathing all of the time, the respiratory system is always at work. You breathe in and out from your nose and mouth while your lungs are the main organs that allow your body to absorb the oxygen you need from the air. There are many other systems in your body and specialized systems in other animals around the world. 

 

What does the system do? We're going to use you as the basis for our explanation of the digestive system. Let's start with eating. You get hungry and you eat. Once you put the food in your mouth, you start to chew and begin a process of mechanical digestion that grinds food down into a pulp.
Your body also starts to release enzymes that start the process of chemical digestion and the breakdown of biological molecules. Most chemical digestion happens in the stomach. The food moves through your digestive system and is eventually broken down into compounds and nutrients that your small intestine can absorb into the blood stream. The material you don't absorb continues into the large intestine where water is removed from the material and then whatever is left can be eliminated at your convenience. That's a decent overview of the process.