"Ben je nog in Amsterdam?"
Translation:Are you still in Amsterdam?
Like Dutchesse722 said, both mean 'still', but in this context, it changes the meaning:
- Ben je nog steeds in Amsterdam? - Are you still in Amsterdam? (?! Why are you not home yet?)
- Ben je nog in Amsterdam? - Are you still in Amsterdam? (I might come to Amsterdam as well, if you are still there...)
Nog steeds definitely means still. Nog in the above sentence also means still, however, it also has other meanings, depending on the context.