The heart is actually a muscle. It's made up of four chambers. At the top are the left and right atria, and at the bottom there are the left and right ventricles. The right side of the heart pumps blood into the lungs, and the left side pumps blood to your organs.
After the blood circulates through your body, delivering oxygen and nutrients, the oxygen-depleted blood enters the right atrium. Electrical signals cause the heart's walls to contract and, as a valve opens, the blood is sent into the right ventricle. From there, the blood goes to the lungs, where it's oxygenated.
On the left side, oxygen-rich blood from the lungs goes into the left atrium. When it is full, the atrium contracts, a valve opens, and the blood is sent into the left ventricle. Another valve allows the blood to flow into the aorta, the largest artery in the body. From there, the blood goes to all parts of your body.
This process is repeated over and over. The valves prevent the blood from flowing backwards into the heart.