Trying to get my head around this VR business

[tpapictures]

Some questions that maybe somebody can help me with. Very late to the party, here.
1. So, I get ANY VR viewer that fits my phone and I just get whatever app and that's that?
2. Is this controller I see necessary and can anything else be used instead of buying one?
3. Other than comfort, will the visuals actually vary much from a cardboard design to a physical headset?

Thanks!