Tree Rings, how do they work?

I want to know how tree rings grow. I know that they are used to tell the age of a tree in years, so ergo they grow a ring every year, but where from? Is new growth in the centre and it grows outwards like a ripple on a pond, moving out from the centre? Or is it from the outside, as new bark grows it forms a layer and becomes the next expansion point, then next season more bark grows, I've seen some really barky trees and its the same bark year to year, I am sure. OR is there a common ground between inner and out where it grows from? Just under the surface, pushing outwards. I grew up in Australia so I am used to Gum Trees, they have a stringy bark that just peels off, you don't really see the tree growing though. Is the bark a ring?