There are two main types of speed and distance monitors for runners: GPS-based models (that use satellites to track your movement); and models that use foot-pods on your shoelaces to measure acceleration and deceleration like a space-age pedometer.
Both can be phenomenally accurate in the right circumstances. GPS models have the advantage of not needing calibration – but they don't work brilliantly in built-up areas or under trees. Pod-based models work wherever you are, though for maximum accuracy they do need adjusting to your own footstrike, and they can be susceptible to extreme changes in underfoot conditions.
Either way, they're loved by increasing numbers of runners – and the models in this top-of-the-range section include heart rate monitoring functions.