The left ventricle is thicker and more muscular because it has to contract to pump blood around the body, compared to just the lungs.
The ventricles have thicker walls than the atria, as they have to pump blood out of the heart instead of just to the ventricles.
Atrioventricular valves link the atria to the ventricles and stop the backflow of blood back into the atria when the ventricles contract.
Semilunar valves link the ventricles to the pulmonary artery and aorta and stop the backflow of blood into the heart after the ventricles contract.
Generally, if there is a higher pressure behind the valves, they are forced open.
The steps to the cardiac cycle are:
1) ventricles relax, atria contract.
The atria contract, decreasing their volume and therefore increasing the atrial pressure. The atrioventricular valves are already open, while are the semilunar valves remain closed. As a result, blood is pumped into the ventricles.
2) ventricles contract, atria relax.
The ventricles contract, decreasing their volume and thus increasing their pressure. This forces the atrioventricular valves shut and forces the semilunar valves open. Blood is pumped out of the heart into the arteries.
3) ventricles relax, atria relax.
The higher pressure in the arteries closes the semilunar valves. Blood returns to the atria, decreasing their volume, increasing their pressure. This forces open the atrioventricular valves. As a result, some blood flows passively into the ventricles (without being pushed). The atria contract and the cardiac cycle starts again.