The tricuspid, aortic and pulmonary valves all have 3 cusps whilst the mitral valve only has two cusps.  From the diagram it is also possible to see that the aortic and pulmonary valves are much smaller than the tricuspid and mitral valves.  The tricuspid and mitral valves are around the same size, whereas the aortic and pulmonary valves are the same size. As we are all unique the size of these valves differ between individuals.

The tricuspid and mitral valve are also known as the right and left atrioventricular valves respectively.   These valves exist to prevent blood flowing backwards to the atria when the ventricles are contracting.  The valves do not stop the blood flowing backwards when the ventricles are relaxed and not contracting.  When the ventricles do start contracting the valves close preventing blood flow in the wrong direction, ie backwards. 

The pulmonary and aortic valves are also known as the semilunar valves and exist to prevent blood flowing backwards to the right and left ventricles respectively.

If the valves cease then one will develop valvular heart disease.