Take a look at this chart/tool. You can also change it to your exact gears.

http://www.bikecalc.com/gear_inches

What it shows is how far the bike moves each time the the crank turns. So a lower number means less distance but not as much effort going in. A higher number means you go further (faster) but you put out more work.

So what this means is an 11 cassette as opposed to a 27 means you go fast and it's hard work - an all out speed effort on the flats or downhill; a 27 means you go slow and easy - what you need for climbing a steep hill.