Most vehicles don't have both shocks and struts, so check your Owner's Manual when the garage tells you that you need to replace both. Some have just struts and some vehicles have only shocks. Shocks are part of the overall suspension and a strut is a complete suspension assembly.
The shocks and/or struts in your car perform two functions, dampening spring oscillation and aiding with ride control. The springs in your car actually absorb road shocks. The shock absorber's function is to dampen the bouncing spring. Without shock absorbers to dampen the spring oscillations from road shock, the ride in your car would be as exciting as the rodeo, about as comfortable, and almost as dangerous.
The shock absorbers keep the car manageable during regular driving, while passing over the road's many little bumps, by keeping the tires on the road rather than bouncing in the air.
Watch for these symptoms:
Worn shocks and/or struts can accelerate the wear of your tires and suspension parts, including the ball joints, steering linkage, springs and C.V. joints. Preventative checkups help prevent these impacts of worn shocks and/or struts, and should be checked every time you have an oil change.
Things to look for:
Shock absorbers last between 30,000 and 45,000 miles, depending upon your car's weight, as well as the amount of time you drive on gravel or icy roads, and the severity of pothole in your area.
You can replace conventional shock absorbers by unbolting the old shocks and installing new ones. Most shocks are bolted to the frame by an upper straight pin, held in place by a single nut. Use a pair of pliers to hold the shaft while you turn the bolt with a box wrench. If you find the two sheet-metal bolts on the bottom frozen with rust, and a wrench won't work, then you may need to torch or chisel them off, or use a nut splitter.
To reduce weight and conserve space in crowded engine compartments, many manufacturers have moved to MacPherson strut suspensions. The strut and damper unit is surrounded by the spring, which acts as a suspension support member. Because of this, mechanics use powerful spring compressors to prevent the springs from suddenly uncoiling. If you lack these tools or experience with MacPherson struts, you should leave this job to your service mechanic.