Steel doors are made of either a polyurethane or polystyrene core which act as a thermal barrier, with a steel skin. The skin is made of steel between 16 and 24 gauge and a wooden frame, “stile”.
When you shop for a steel door, make sure it has all the windows and pet doors you need, as it will be very hard to make them later.
Steel doors come primed, ready for paint. They also sometimes come with vinyl PVC, made look like wood grain.
A great thing about steel doors, is that they have a fire rating. But they are heavy.