Not many people realize that stainless steel is not a particularly good conductor of heat. What this means is that hot spots can occur so if you are cooking a steak for instance you may find that part of the steak is cooked while other parts are undercooked. In other words your foods are cooked unevenly.
To overcome this problem, stainless steel pots and pans are often lined with aluminum or copper as both of these metals are quite good at conducting heat. However, because both of these metals are relatively soft and can react with certain foods, they are usually sandwiched between layers of stainless steel.
Although both copper and aluminum are good conductors of heat, copper is better because it reacts quickly to changes in temperature. This is why you often see professional chefs using copper cookware. It gives them greater control over their cooking.
The disadvantage of copper however is that it is more expensive than aluminum.
So if you are on the lookout for new stainless steel cookware then ensure that the base contains either aluminum or copper.