Premium stainless clad cookware is made by bonding 5 layers of metal together to capture the best qualities of each.
In the "premium" market you have things like tri-ply, 5-ply, copper core, etc. The outer layer of the metal is premium 18/10 stainless steel. Stainless steel provides durability, induction compatibility, and a non-reactive surface.
The middle three layers are alternating layers of pure aluminum and aluminum alloy, which is a combination of metal known for its heat conductivity and responsiveness.