A strut is a rod or tube that is acted upon by forces, either side forces or compression forces. The strut assembly integrates the shock absorber and a coil spring.
Because struts keep the vehicle from bouncing around when driving over certain road conditions, they tend to wear over time. When the strut begins to fail, it will cause handling problems an swaying and dipping and uneven tire wear. Some manufacturers recommend that the strut be replaced every 40,000 miles, while other recommend replacing it when it begins to fail.
The strut is normally covered under the new car warranty of a vehicle is the failure is due to to manufacturer defects or workmanship. Typically the replacement of a strut is covered by an auto extended warranty under certain levels of coverage.