The examples above are correct, but can still be confusing, so I'll try to clear things up a bit.

Most people are familliar with Dual Rates. They are easy to understand, if you set your dual rates for 50%, you'll get 50% of full control throw when the stick is at 100%. By that logic, you'll get 25% of full control throw when the stick is at 50%, and so on.

The good thing about dual rates is that if you have a "Twitchy" airplane, using dual rates will smooth it out for you.

The bad thing about dual rates is that if you get into a bad situation, you only have 50% (Or whatever value you have it set to) to get out of trouble. (I've seen several crashes caused by someone forgetting that their dual rates were on and attempting a loop!)

Expo is the best of both worlds. The stick movement will result in less control throw at the beginning of travel, and more near the end. So for the first half of the stick travel (Where you need the controls to be "Soft" ) you get less movement of the surfaces, but when you crank the sticks, you still get full surface deflection.