Their bottoms have three layers: a middle ply (generally aluminum) sandwiched between two stainless steel ones. The purpose of this design is to give the cook the best of both worlds by eliminating each metal's disadvantages.
The aluminum layer cannot become discolored, nor can it color or flavor foods, because it is completely enclosed within the stainless steel. The upper stainless steel layer does not have the hot spots that are common in 100 percent stainless steel pots, because by the time it reaches that stainless steel tier, the heat from the burner has been more or less evenly diffused by the aluminum (which is, unlike stainless steel, an excellent conductor of heat). And because the pan's entire metal surface is stainless steel, it has an attractive shiny finish and is easier to clean. Still another bonus is that multi-tiered construction has much the same effect on the pan's bottom that it has on plywood: The possibility of warping is decreased.