Contrary to common belief, shock absorbers are not what absorb shock in your car. The springs take care of that. Shock absorbers are what help stop the springs from oscillating after a shock is absorbed by the vehicle, and they cannot be repaired---only replaced. Without them, the springs would continue oscillating long after the shock is absorbed.
The Shock Absorber
The shock absorber itself can also be called a "damper," due to its purpose; the dampening of shocks absorbed by the car's springs.
How it Works
Shock absorbers work primarily because of hydraulic fluids. They have pistons which attach to supports on the vehicle and hydraulic fluids which help to control oscillations in the car's springs.
Shocks tend to go bad over time. According to basiccarrepair.com, "deterioration is gradual." You can't expect to be able to accurately predict when shocks will go bad because the amount of wear they receive is a huge factor in determining their life expectancy. In vehicles where the shocks are used often and excessively (such as off road vehicles), they are likely to wear out sooner.
Heavy Duty vs. Standard Duty
In the world of shock absorbers, you may come across the terms "heavy duty" and "standard duty." At first, you may believe this refers to their life expectancy, but don't be deceived. Heavy and standard duty refer to the purpose for which the shock absorbers were created. Heavy duty shock absorbers are made to absorb more serious shocks and can be found on more heavy duty vehicles. Standard duty shock absorbers are more likely to be found on your average car.
More on Life Expectancy
According to basiccarepair.com, the biggest factor in your shock absorbers' life expectancy is the kinds of roads you drive on. Smoother roads will lead to less wear and tear in your shock absorbers. And, since there is no truly accurate way to determine whether your shocks are going to last, it is recommended you get them inspected once every six months.