To find out how all these things work together for good, have a look at this article I wrote for In Britain, official organ of the British Tourist Authority.
For a more visual tutorial, the anatomy of a typical roundabout, look here...
To read about the Whirlies Roundabout in East Kilbride, look here...
To peek at a most amazing "Magic Roundabout" look here . . . (not for the feint of heart).