A heart rate monitor gives you info on how strenuous your exercise is, as well as how long you need to recover after it. Some models also incorporate activity monitoring, which helps you keep a good general view on your activity levels.

The model range is wide: the simplest models are affordable and still give you the most important data: your heart rate, enabling you to control how much to exert yourself. The smartest models, again, are like coaches, giving you analyses on your performance and the state of your body.