Active suspension makes your car handle better at the flick of a switch.

Active suspension (first used in racing cars) is a relatively new development in the motor car. Essentially its job is to deliver better road handling characteristics for different driving conditions.

Active suspension is a feature that when functioning correctly has the effect of what amounts to the handling characteristics of a sports car with the feel of a big luxury car. This dual role is achieved by using a control system that will (when provoked by hard driving or when manually switched) adjust the suspension to give a firmer ride and more "feel". If you've got it in your car there may be a switch close to the gear lever that will enable you to manually switch the suspension from a soft to a hard ride setting or "cruise" to "sport". Some systems are also controlled automatically, allowing you to simply concentrate on enjoying the car. In most cases if there is an active part in the suspension then this will often mean there is a hydraulic oil reservoir (although some operate by air pressure) under the bonnet which may need level checking occasionally.

There are several systems in use but the more popular ones such a Citroen work by using hydraulic pressure to control the shock absorbers, car's ride height and influence the behavior of the car. Sensors automatically monitor vehicle road speed, steering wheel position, acceleration and body movement and relay this information back to the central control unit which will adapt the suspension settings to give the most suitable ride and car control at all times.

In its automatic version this is all done without you really being aware of too much. 

The car approaches a corner at a high speed and starts to produce  body roll. The roll sensors will notice this and the active suspension system will activate and provide an opposing force to minimize the cars body  lean,  increasing the car's ability to corner more efficiently and safely.

It is important to remember that active suspension systems are designed to allow better handling and a more comfortable ride. They can't do much about a situation that is beyond their capabilities and must not be considered fool-proof. 

If the car is taken beyond its limits, like anything else, it will more than likely result in an accident.

If the system was to fail in any way there is very little chance of it becoming dangerous to use. Fail-safe functions are built in, for example the shock absorbers would revert to the default softer setting, allowing you to drive the car happily until the problem can be rectified.

However if the system does fail, it would almost certainly be something that is beyond the scope of the average DIY mechanic. Because electronics and hydraulics are extensively used to operate it, any problem is best left to your local garage who will have the ability, training and tools to diagnose and rectify the fault.