A fjellstue - literally mountain parlour, plural fjellstuer - is a staffed lodge in the Norwegian mountains. Someone lives there and runs it. There are made beds, a dining room, hot meals served at set times, drying rooms for wet clothing, and usually a wood-fired stove in a common room. Many are family-run and have been for generations. The word carries no camping connotation whatsoever.
This matters because English flattens two very different things into one word. A visitor reading about hut-to-hut skiing in Norway will find both self-service DNT cabins - where you let yourself in with a standard key, cook from a stocked pantry, and settle up on the honour system - and fjellstuer, where dinner is at seven and your bag arrived by car. Both are called huts in English. They are not remotely the same holiday, and the difference decides whether a week in the mountains suits you.
The practical distinction is who carries what. On a self-service hut route you carry a sleeping bag liner, some food, and everything else for the day. Between fjellstuer, luggage is often moved by vehicle while you ski or walk with a day pack. The distances can be identical; the demand on you is not.
A fjellstue is also not a seter, though the two are often neighbours and the names overlap in practice - several lodges that operate as fjellstuer today began as summer farms and kept the old name. Where a seter was built for livestock and a season, a fjellstue was built for travellers and stays open for them.
On the Rondane plateau above Gudbrandsdalen, three of them sit within a short day of each other, which is why that stretch works as a first multi-day trip: you move between staffed houses, and the shortest transfer is under ten kilometres.