Both are "clad" cookware (stainless steel with aluminum cores), but the "ply" refers to layers. 3-Ply: Typically has a stainless-aluminum-stainless sandwich. This is the professional standard. 5-Ply: Adds extra layers, often stainless-aluminum-stainless-aluminum-stainless. More metal = more thermal mass. https://knivesarea.com/best-pots-and-pan-set-non-stick/