It is hardly surprising that nobody outside Norway knows much about römmegröt, which is porridge enriched with sour cream (römme).
Sour cream is a traditional ingredient in Russian, East and Central European and German cooking, and over the last fifty years has become a staple in Western Europe, North America and elsewhere. Smetana is a mixture of sour and fresh cream and is probably not named after the self-interested Czech composer who knew which side his bride was bartered.
Sour cream used to be be made by letting fresh cream sour naturally but nowadays is made by pasteurising and homogenising light cream and inoculating it with a pure culture of selected bacteria, keeping it warm to facilitate their growth and then re-pasteurising it to stop the process. This is not the same as crème fraîche.
I had intended to write an authoritative survey of cream and crème in all their variations, covering French and English official regulations (e.g. crème fraîche must contain at least 30 grams of fat while crème légère need only have 12), but I really couldn't be bothered to look up all the EU regulations which may have superseded them, or to explain the French Chantilly, fluide, épaisse, à café, and aigre and what we call double, whipping and clotted.
And then there are all the other French crèmes: anglaise, patissière, au beurre, frite, renversée, brûlée and caramel, and the English creams: crackers, soda, puffs and eggs.
Easier perhaps merely to quote the OED's definition of cream, though it is singularly unappetising:
The oily or butyraceous part of milk
[ME. creme, creem, creyme, a. F. crème, in OF. cresme, Pr. cresma. By etymological conjecture crème, cream, was in 16th c. referred to L. cremor, and latinized as cremor lactis, crema lactis.]