A normal heart has two separate sides with different roles working closely together. The right side receives blood from the body and pumps it to the lungs (to be topped up with the oxygen we breath in), and the left side receives blood from the lungs before pumping it around the body. Each side is made up of two rooms or chambers, each of which has a specific role.
The top chambers are called the Right Atrium and Left Atrium; these collect the blood returning from the body and lungs respectively.
The bottom chambers are called the Right Ventricle and Left Ventricle; these are the two pumping chambers, pumping blood to the lungs (blood to be topped up with oxygen) and around the body respectively.
Separating the heart chambers are valves made of tough flexible tissue which open and close in response to pressure changes within the heart chambers. These are the doors controlling the smooth flow of blood through your heart ensuring it is working as efficiently as possible, i.e. moving forwards, not backwards.