Mnh And NH

Hello,

I am new here and want to by some stamps on ebay.com.

I am from Germany but could not find out what the difference between MNH and NH is?
What is XF NH? I know its extra fine, but does NH not indicate that the stamp is in its original state as it was from the post office? Was it then extra fine from the beginning?

Is there a difference?

Thanks for any helpful clarification on these phuilatelic terms.

Best Regards
Stahli

NH could for example have a short perf, crease or damaged gum whereas MNH wouldn't (or at least shouldn't).