The smallest cog on the cassette is a harder gear to pedal. The largest cog is the easiest gear to pedal.

On the front chainring, it's the opposite: the small chainring is easier to pedal and the large one is harder to pedal.

When you combine the small cog in the back with the large chainring in the front, that's the hardest gear (and may skip because it puts the most stress on the chain). When you combine the large cog in the back with the small chainring in the front, it's the easiest gear.

Sheldon Brown has alot of very useful information. www.sheldonbrown.com