"Het is koud in het noorden."
Translation:It is cold in the North.
"Noord" means northern and is only used in geographical names. It is either used as prefix (usually with a hyphen), or after a geographical name:
- Noord-Amerika - North America
- Noord-Holland - name of the province that Amsterdam is in.
- Amsterdam Noord - the northern part of Amsterdam.
There is also the adjective "noordelijk", with means northern in all other contexts. "Noordelijke wind" - northern wind.
"Het noorden" is a noun, you will encounter it much more often than the above words. Aside from the cardinal direction (the north ), it is also used as a direction of motion:
- Ik ga naar het noorden - I go North.
- Hij woont in het noorden - He lives up North (or: in the North)
Also, I believe in the context of Amsterdam Noord, you can use the sentence "Ik woon in Noord".
I'm not sure if it can be used for all places, or it's just an Amsterdam thing.