The main difference between the atria and the ventricles are the areas that they hold in the human heart. The two upper chambers are called the atria, while the two lower chambers are called the ventricles. The atria are in charge of getting the dirty blood before allowing the blood to get into the ventricles.
Another difference is the thickness of the walls of the heart. The walls of the ventricles are known to be thicker while the walls of the aorta are known to be rather thin. The ventricles may sometimes be connected to the brain cavities while the aorta will be completely in charge of the inner chambers of the heart.