Quote Originally Posted by Kano
Oh dear -- I'm not certain Porsche and Volkswagen are apples and apples... Especially confusing is when the Dodge looks just like the Chrysler, you know, and the derailleur thingies look alike, but why does the one on the blue bike make the bike cost so much more than the one on the red bike? See what I mean? (I'm still learning about why shorts aren't shorts aren't shorts!)
The difference between two similar bikes may be the components. The wheels can also bring a big difference in price, just like a carbon versus a steel fork, etc.

With regards to the components (which are Shimano or Campagnolo and very seldom SRAM - personal preference mostly), the difference will be reliability, durability and precision.

In the Shimano line, the order goes like this, from entry-level to top-notch
Sora
Tiagra
105
Ultegra
Dura-Ace

Campagnolo has a similar hierarchy with names like Chorus and Record and Veloce that I never seem to be able to put in order.

There are a few things below Sora but these are the most common in quality road bikes with Shimano.

Basically, the higher you go, the less often you will have to adjust your transmission, and the more precise it will be. I often joke that my Ultegra-clad road bike shifts just by thinking about it.