The ventricles are the two lower chambers of the heart.
In Our Own Words
The heart is divided in two halves by a wall or septum. Each half of the heart has a smaller upper chamber (atrium) and a larger lower chamber (ventricle). The ventricles receive blood from the atria. The right half of the heart is involved in collecting oxygen-poor blood from the body and sending it to the lungs where it becomes enriched with oxygen. The left half of the heart is involved in receiving blood from the lungs and pumping it to the body, where the oxygen is depleted by the tissues.
Failure of one ventricle can lead to failure of the other, since they work together. When not contracting, each ventricle should relax enough so that it can fill up properly.