two sets of gears on a bicycle?

[Matthew G]

there are two sets of gears on a bike. the set on the right are for pedal resistance, but what are the set on the left for?

 

[Bob A]

The gears on the left / front give you a range, the ones on the right lets you fine tune the cadence / speed you need.

Left 2 and right 1-7 for normal ridding.
Left 1 and right 1-4 for hills
Left 3 and right 5-7 for going fast and down hill.

http://bicycletutor.com/gear-shifting/

http://sheldonbrown.com/gears.html