Nøkken is a man, sometimes beautiful, most of the time horribly ugly. He lives in still water (no rivers or streams, those are the homes of Fossegrimen), usually at the deep, sometimes under water lilies, which we call Nøkkeroser. Sometimes he showed himself to people as a white horse and tricked them to ride him. The moment they were on his back, he sprinted towards the water where he drowned his rider. He can also take the shape of a wooden log, to fool people. He's deadly, but not necessarily evil, like the trolls.
We never use nøkk in plural, as they are very solitary / lonely creatures. He's either called nøkk or nøkken ("the nøkk").
Nisser is a part of the crowd we call "de underjordiske" ("those who live under the ground"). Every farm has their own nisse, and how well you treat yours decides how well he treats you. There are stories of nisser who's killed cattle and led families into starvations for snubs like not greeting him properly, or not giving him enough butter in his porridge. Some families still out porridge to the nisse on Christmas eve. It's important that you give him enough butter in his porridge, or he will be angry!
I remember one story where a farmer got himself a new wife, who refused to give the nisse porridge. The nisse in return made his cattle sick and his farm fall apart. The moment the farmer left his wife, all was put right.
There are also stories where nisser makes sure to protect the farm from storms or starvation because of previous acts of kindness. But for the nisse, it doesn't matter if you're good or bad. What's important is if you're mean to him.
Plural for nisse is nisser.
I also have a blogpost about Norwegian trolls.