A dive computers primary function is to work as a device to track a diver’s decompression status. A dive computer monitors your depth and time throughout the dive. Its microprocessors plug information into a mathematical decompression model that estimate how much nitrogen or other inert gases have gone into solution in your body tissues. The computer is constantly updated and it displays how much time you have left based on your current depth, before you reach the maximum allowable limits. If you exceed these limits, the computer displays the depth at which you must stop your dive, and for how long, before you're allowed to surface.
Computers differ slightly from model to model and from manufacturer to manufacturer, but most of them have the same principle characteristics. Most dive computers monitor: time, date, temperature, depth, nitrogen uptake, ascent rate, no decompression limits, and safety stop timer. Better computers will have those basic features plus other great features, such as being air integrated so it can read how much pressure is left in the tank. That gives more accurate nitrogen uptake readings and allows for the computer to give alarms for different pressure reading and the computer can warn the diver to end the dive because the air tank is getting low. This also gives the computer a very accurate readings of how much dive time is remaining based on many factors.
Also, new computers are using Bluetooth to send the dive profiles and logs directly to computer and smartphones. Bluetooth and air integration are just a few of the upgrades of better computers.