A ventricle is a hollow area or cavity within an organ usually filled with some kind of fluid. Our brain has four interconnected ventricles and the fluid that fills them is called cerebrospinal fluid.
Cerebrospinal fluid is a clear fluid that has several important functions in our central nervous system and you might be wondering where this fluid comes from. Well, almost every part of the ventricular system, that we're going to see in the full version of this video, contains a choroid plexus which includes epithelial cells that produce the cerebrospinal fluid.
So as I just mentioned there are four ventricles within the brain - two lateral ventricles (the left and right lateral ventricles), the third ventricle found near the central of the brain, and the fourth ventricle located deep to the cerebellum.