I will try to help you with your questions about the heart and how it functions. As you mentioned the heart is a large major muscle and it basically acts as a pump. The "beating" of the heart is the muscle contracting and squeezing the blood from the heart through the blood vessels to the rest of the body. A very simple explanation is that the function of the heart is to deliver fresh oxygenated blood to all parts of the body and to return the used blood to the lungs for more oxygen. This cycle goes on with out stopping for a long as the organism is alive. It is really very much more complicated that this but if the other organs (brain, liver, etc.) do not get a fresh supply of blood they will cease to function and the person will die. This type of pump (the heart) has worked out to be the most efficient organ to move blood and it does a remarkable job. Here is a good web site for you to visit for more info on the heart.
http://www.heartinfo.com/

In order to understand why the heart is vital to our lives, we have to look at what blood does.The main function of blood is to deliver oxygen and fuel (sugar) to each cell in your body. (What is a cell?) Without a constant supply of oxygen and sugar, the cells die.

One celled animals live in places where they can pick up oxygen and food from the fluid (like water) that is all around them. As animals became more complicated, cells were next to other cells instead of being next to the supply of oxygen and food. Then those things had to be delivered to the cell. Think about your own home. It is probably not next to a farm, a lake, and a power plant. Food, water, and power have to be delivered to your home. All the cells in your body need the oxygen from the lungs and the sugar from your food. They also need to get rid of their wastes just like you need to send your garbage somewhere else. So blood delivers what the cells need and picks up the cells' wastes. (What waste do you breathe out?)

Blood can't move by itself. That's where the heart comes in. You are right that the heart is made of muscle. The heart is the pump that moves blood to the lungs to pick up oxygen, the to the cells to deliver it. Other blood is moving by your digestive system to pick up sugar and deliver it to hungry cells. On other trips, your blood goes to the kidneys and liver to have the "garbage" filtered out. Without the heart, the blood would be like a whole fleet of delivery and garbage trucks that were out of gas. They might have important things to deliver, but they can't move. This is what happens if your heart doesn't beat.

Your heart works like a bike tire pump or a squeeze bottle. It contracts (gets smaller) so the blood is forced out. When it expands (gets bigger), new blood rushes in, just like air will rush into the squeeze bottle or pump when you stop squeezing or raise the handle again. Valves keep the blood from backing up. (What is a valve?)

The American Heart Association has a good information on heart disease.
They're at: http://www.amhrt.org/.

Enjoy your heart research.