whats the difference between gears?

[Grog]

To clarify: Shimano's road line is--from low to high end: Tiagra, Sora, 105, Ultegra, Dura Ace.

Actually according to Shimano's web site it's, from low-end to top-end:

2200
Sora
Tiagra
105
Ultegra
Dura-ace