In this post Tony explains the features and functions of various bicycle computers. Although they are actually tiny computers you will some times hear them referred to as speedometers or cyclometers.
As you’ll see in the video they start out from a basic version which has a wire attached to a pickup that gets information from your wheel to a completely wireless version with lots of extra features.
If you plan to use your bike for exercise you might want to get one that has a heart rate function so you can exercise at the ideal aerobic rate for the best fat burn. It wasn’t that long ago that features like these were just not available.
After watching the video you should have a pretty good idea which one would be best for you.