The Heart is an organ that pumpes our blood thought the blood vessels and helps us to breath.
heart diagram animated
heart diagram animated
external image aniheart.gif The heart is a muscle. It is located in the left middle of the chest and you can compare it to your fist. The heart pumps the blood which carries oxygen and nutrients to your body. The right side of your heart is takes the blood in and give it to the lungs and the left side pump the blood to the rest of the lungs and body. When your heart get full of blood it removes it tighens which creates a heart beat.
The heart has four different chambers. On each side, there are two different chambers also on the top and bottom. The two chamber on top of the heart is called "Atria" but if your refering to only one chamber on the top, then it's called "Atrium". The atira fills the blood coming back to the lungs and our body so it helps our body to breath and make us stay alive. Also, the two chamber on each side is called right atrium and left atrium. The two chambers on the bottom are called "ventricle" that squizes the blood to the body and the lungs. The heart has left ventricle and right ventricle. In the middle of the heart, there is a thick wall called "septum" that seperate the left side and right side of the heart. The atira and the ventricle work together. For example, the atria fills the blood then it will dump it in to the ventricle. Then the ventricle squeeze while the atria refills and get's ready for next same step. The heart has a two valves. One of them is called "mitral valve" and other one is called "tricuspid valve". They let your blood flow from the atria to ventricles. Also, the other valves are called "aortic valve" and the "pulmonary valve" that controls the blood flow from our body. Those valves help our blood flow right. The valves open the blood to move forward then they closed it fast to keep the blood coming backwords.

http://kidshealth.org/kid/htbw/heart.html