Upstairs and downstairs: atria and ventricles
The four chambers come in two pairs. The two atria sit on top and act as the heart's waiting rooms: they collect blood returning from the veins and gently top up the chambers below. The two ventricles sit underneath and do the heavy pumping. Think of atria as filling chambers and ventricles as ejecting chambers.
Naming them is just a matter of side plus level. The right atrium takes oxygen-poor blood from the body; below it, the right ventricle pumps that blood to the lungs. The left atrium takes oxygen-rich blood from the lungs; below it, the powerful left ventricle pumps that blood to the entire body.
The walls that keep the sides apart
What stops oxygen-rich and oxygen-poor blood from mixing? Two internal walls, called septa. The interatrial septum divides the two atria, and the interventricular septum divides the two ventricles. In a healthy adult heart these walls are complete, so the left and right streams never touch.
When a septum has a hole in it — present from birth — blood can cross from one side to the other. You don't need the details now, but it helps to know that a gap in these very walls is the basis of common congenital problems, which is why naming the septa matters.
Why the left ventricle is so much thicker
If you slice across the two ventricles, the left wall is roughly three times thicker than the right. The reason is purely mechanical: the left ventricle must push blood against the high pressure of the whole body, while the right ventricle only pushes against the low pressure of the lungs. More pressure to overcome — what we call higher afterload — means a thicker, stronger muscular wall.