A means of hydraulically assisted steering. A belt-driven power steering pump creates system pressure. The pressurized fluid is then routed into a cylinder that helps push the wheels one-way or the other when the steering wheel is turned. The two most common power steering complaints are noise and leaks.
A slipping drive-belt on the power steering pump can produce a loud squeal, especially when turning sharply. A bad valve or bearings in the pump itself can make a growling noise. Leaks most often occur at hose couplings or on the power cylinder seals. In power rack & pinion steering units, internal leaks can be a major problem (which require replacing the entire unit with a new or rebuilt assembly).
The only required maintenance for this system is to check the level of the power steering fluid periodically. If low, check for possible leaks, then add fresh fluid to the pump reservoir. Running the system low can ruin the pump.