The hero is the New York name for the long-roll sandwich, and the name is the most local thing about it. Strip the word away and the founding principle is the same one the hoagie, the grinder, the wedge, and the sub all run on: a long roll, split, layered the full length of it, and built so every bite holds the whole sandwich. In New York that roll is typically a length of Italian bread with a real crust, and the crust is the defining engineering. It has to have enough structure to carry a long, heavy, often wet load without folding in the middle, and a tender enough interior that it does not fight the filling.
The craft is in the layering and the dress, and it changes with the build. The Italian hero shingles cured meats and provolone so each bite gets all of them, then uses oil, vinegar, oregano, and shredded lettuce as a system that seasons and lubricates without dissolving the bread. The hot heroes turn the roll into a vessel for a saucy filling: the chicken or veal parm and the meatball hero depend on a sturdier crust and a quick toast so the bread does not surrender to the sauce before the last bite. The eggplant parm runs the same logic meatless. The shared rule under every version is that the roll is a structural component chosen for the load, not a wrapper.
The variations are mostly the same idea spoken in other towns' accents. The Philadelphia hoagie, the New England grinder, and the Westchester wedge are regional names for what New York calls a hero, with small dressing and bread differences that locals defend. Within New York the line runs from the cold Italian to the hot parm and meatball builds to the tuna, turkey, and chicken-cutlet versions that keep the architecture and change the filling. Each of those is a codified build with its own rules, and each deserves its own article rather than being crowded in here.