Originally Posted by Laggard
11-23 = a rear cassette with 11 teeth on the small cog and 23 on the largest. 39/53 means a large chainring of 53 teeth and a small chainring of 39.
More teeth on the cassette cog equals easier pedaling. It's the opposite in the front.
If your going to ride a lot of hills you might want to consider a 12-27 unless you are pretty strong and a good climber.