The Wikipedia entry on heart rate is actually useful.

And Chinarider is right: the 220-age formula is inaccurate. There are better formulas but they also lead to an average. I'm 43; according to the formula my MHR should be 177, but I've measured it at 193.

Resting heart rate (RHR) is easily determined. Don't just enter 70; instead, hook up your heart rate monitor, lie down and relax, and wait a few minutes. Then turn on the timer. Stay calm and still. After a couple of minutes, turn it off again. Use the average HR as your RHR. Don't do this after you've been working out, since your heart takes time to recover. I habitually take my pulse first thing in the morning; I've noticed a wide variation in RHR, from 52 if I am well rested to over 60 if I slept poorly, am stressed, or overindulged on food or booze the previous evening.

Heart rate reserve is simply the difference between MHR and RHR. Some aerobic training methods use RHR plus percentages of HRR, rather than percentages of MHR.