The things makes the Japanese sword(Nihonto) unique is the way in which the blade is constructed by different types of steel together to forge. these lamination give the blade durability and strength. and these laminations methods give a hard cutting edge and softer spine make sword resistence to breakage.
Maru -no lamination, it is forged only one type steel.
Sanmai -three laminations, just like sandwich, core steel harder than both side steel. it is most common lamination use in forging Japanese sword.
Honsanmai -it is similar with sanmai. forging use 3 types steel. back steel using soft steel.
Kobuse -it is means wrapping steel, outer side hard steel wrapped core soft steel, it is common method to forging Japanese sword.
Soshu kitae -Seven layers constructions method. it is very complex construction, and also very expensive.