Basic indications
While underwater all dive computers will tell you the current depth, the maximum depth reached during the dive, the time you spent underwater, the remaining time within the safety curve or the depth and duration of the next required deco stop; after the dive they will indicate the no-fly time, the surface time and, usually, the total desaturation time left. All computer have some form of visual and/or audible alarm to indicate possible dangerous situations such as the necessity for a deco stop or an excessive ascending time.
Most computers also offer other information such date and time, water temperature, some visual indication of tissue saturation and of the ascending time. Dive computers often allow you to 'plan' you next dive by taking into consideration your residual nitrogen time and showing how the safety curve is modified accordingly. In many cases they also allow a dive simulation that will help you in understanding how the computer will react to different dive profiles. All computer can take into consideration the effect of altitutude diving either by adjusting automatically to the altitude conditions or by allowing you to do so. They will also maintain a log of your dives by registering some statistical parameters as well as the detailed profile of the last immersions.

Extended functions
In reality modern dive computers give you much more than these basic functions. Air integrated computers, for instance, are gaining popularity among divers. Such computers constantly measure the pressure inside the tank, either by connecting to the high pressure hose or through a hoseless radio transmitter attached to the first stage, and can indicate you the remaining pressure and the remaining air time given the current breathing rate. Alarms can warn you against an excessive air consumption or the reach of the air reserve limit.
With the diffusion of Nitrox diving all the main producers have released computers with Nitrox capabilities. They all allow you to plan and manage enriched air dives and give you indications about certain conditions related to partial gas pressure that may arise during the dive such as, in particular, an excessive oxigen partial pressure.

A quick check list of features to consider before buying
The first choice when considering the purchase of the dive computer is wether you need and/or want an air integrated computer. In many case a wrist computer with its compact size may be more convenient, but if you prefer to have the computer closely monitor for you the air reserve and tell you the remainig air time, then an air integrated computer may be the most obvious choice. In this case you must decide if you prefer to have a consolle mounted instrument or to keep the computer on your wrist using a hoseless transimitter. If you are considering Nitrox diving than it could be wise to choose a model among the intruments with Nitrox capabilities that, in any case, are also perfectly suited for normal air dives. Features like display visibility, auto switch-on and switch-off, possibility of download of dive logs to a desktop PC and backlit illumination are also important in your choice that, in any case, must be made primarily based on the type of dives you usually make.