Classic MapMyRide has a pretty good calculator for determining one's heart rate zones if you know your maximum heart rate, or you can accept the calculator's formula for maximum heart rate.

http://classic.mapmyride.com/calculate_mhr

There is a good explanation of each zone as pertaining to cycling.

The calculator's formula for calculating the maximum heart rate actually works for me. At my age and based on knowing my heart rate at maximum exertion, I estimated my maximum heart rate to be between 165 and 170, and the calculator states 169, so right in the range I thought it was.