Until you get a heart rate monitor, you measure your effort by time or distance. With a heart rate monitor, however, you can now get a better indication of effort. And it comes from the heart. Literally. Your heart will tell you how hard you are exercising, how fit you are, and how you are recovering. Knowing these things will make your training more effective.
A heart rate monitor consists of a watch unit worn on the wrist and a chest transmitter strap. The watch can be worn as a regular sports watch, and when not connected to the transmitter strap, will still give you time of day, alarms, stopwatch functions, etc. The chest transmitter senses your heartbeat and constantly transmits that information to the watch.
There are some units that don’t use a chest strap. These normally measure the pulse when the user places two fingers on each of two buttons on the watch face. Pulse is a decent measurement, but does not give you the same level of continual information a chest strap will.