What is Nordic?

"Nordic skiing" and "cross country skiing" are just two different names for the same sport (not to be confused with "Nordic combined", where skiers also do ski-jumping).

Nordic skiing is a totally different sport from downhill/alpine skiing or park skiing. It is an endurance sport, involving totally different movement, equipment, athletes, terrain (Nordic is flat, uphill, downhill), clothing, etc.

A key part of skiing is obviously the snow. To train, the more time ‘on snow’ the better. Nordic skiers also must adjust to conditions, keeping themselves warm enough and prepared. The preparation for skiing heavily relies on waxing one’s skis. Nordic skiers at almost every level wax their own skis, ad thus make use of a table to clamp the skis on as they iron on and then scrape and brush off the variety of wax needed for the given snow and weather conditions. 

Why does it look different at different times?

Just like swimming, there are different types and different things you do within each type, so don't get confused why it looks different:

1. Skate skiing

   1. Pushing legs out to side, both poles land at the same time

   2. You start skate skiing all very close together so you have to just use your arms at first

   3. Skis are totally slippery on bottom

2. Classic skiing

   1. Like running on skis

   2. Skis have a sticky part in the middle so when you press down, you get grip and can push forward

   3. Two things you do while classic skiing

      1. Striding: like running: one pole& one ski forward at a time

      2. Double: poling: both poles down at same time, bending forward

Beyond that, I'm gonna suggest youtube videos of Olympics/World Cup and  let the pros describe it here: https://www.worldloppet.com/what-is-cross-country-skiing/