can someone explain concisely how the US system worked and how it has/will change?

Am I right in thinking that you needed to be insured, which then kicked in if you fell ill?

Or could you just go to any Dr and pay to be seen, like a shop type thing?

What happens if you say got run over, but don't have insurance or any money - do you still get an amberlamp and that?

(and yes I could google, look on Wiki, BBC etc, but I prefer the great knowledge of FTR!)