There's a ton of info you can find, I found about 10 good links just googl'ing "Target heart rate"
It's very dependent on your age, size and how in shape you are, but you can easily find a chart and compute your specific target heart rate
What I've always read & been told is that there are basically 2 target heart rates, one for cardio and one for 'fat burning'. The fat burning target rate will be lower than the cardio heart rate and is meant for longer activities/workouts (eg Walking or light jogging for an hour or more). The cardio heart rate is meant to be your target heart rate for more intense cardio workouts (usually 45 min's or less), eg stair stepper's, elliptical trainers, ...etc.