I've google'd and researched but I still don't quite get the difference between the Shimano Ultegra, Tiagra, 105, Sora, etc.

What exactly does the other do better? I've seen 2012-13 bikes with Dura Ace and wonder, "why didn't they put the newest one available?"

Sorry if this question has been asked or I posted in the wrong section.

Cheers.