"теперь" - "now as opposed to before". It means that some condition/state/action you're talking about or just implying have changed. А теперь пойдем! (you were waiting for something to happen. And didn't think it's a good idea to move before it's happened. Now it's happened, and 'let's go now') Раньше я жил в России, (а) теперь живу в Австрии. Раньше я жил в России, (a) сейчас живу в Австрии. (the same meaning, but the former accentuates the fact you've moved, the 'opposition' of before and now. The latter just lists two facts. Adding the conjunction "a" you provide the same element of 'opposition')

Note, that you may nearly always use "сейчас" in the same situations as "теперь" and it'd sound natural (provided that the context points on the 'opposition' obviously enough.) I mean: if you do mean "as opposed to" and you think that everybody understands it - feel free to use сейчас. Теперь is just an option, no more:)