Hi - this is what I found on another web-site.

A group is comprised of brakes, hubs, chain, cassette, bottom bracket, crank, derailleurs, shifters and headset (sometimes a seatpost is included, too).

As you spend more money, parts get lighter and bearing quality (bearings are what the hubs, headset, pedals and crankset spin on) improves. Higher-level components shift and brake slightly better, too -- though even entry-level braking and shifting is exceptional on modern systems.

So, how do you decide what to buy? It comes down to your price range and which group offers the features you want (i.e. weight, number of gears, appearance, quality). Usually, you can narrow it down to a couple of groups. And, at that point, a great way to decide is to ride and compare. If you can feel a difference in braking and shifting, go with the bike you like better.

Basically the 105 is cheaper than the ultegra which is cheaper than the dura-ace. Ultegra is race quality whilst dura-ace is one step above that.

Hope that helps!