The heart is a muscle, and when it contracts or beats, it pumps blood out. The heart contracts in two stages. First, the right and left atria contract at the same time to pump blood to the right and left ventricles. The ventricles then contract together to pump blood out of the heart. The heart muscle relaxes before the next heartbeat to allow blood to fill up the heart again, since it must be filled with blood to pump. An average heart can pump 2.4 ounces of blood per heartbeat, or 1.3 gallons per minute. Your blood vessels act like pipes to carry the blood to and from the heart, distributing it through the body.
In order for blood to flow, it has to have a pressure difference, since blood -- like any other fluid -- flows from the high pressure to low pressure, just like a waterfall. That's why the pressure in the left ventricle is the highest, followed a slightly lower pressure in the left atrium. The pressure in the right ventricle is lower than the left atrium, but still higher than that of the right ventricle. So the left ventricle, with its high pressure, is able to push blood with sufficient force to send it through the body all the way down to the toes, with enough pressure left to bring it back to the right atrium and repeat the cycle.
The right and left sides of the heart have separate functions. The right side collects oxygen-poor blood from the body and pumps it to the lungs, where it picks up oxygen and releases carbon dioxide. The left side collects oxygen rich blood from the lungs and pumps it to the body so that the cells have the oxygen they need to function properly.