Taps have traditionally had washers in them. They wear out periodically and need replacing. This is usually enough to fix a leaking tap. However, after years of use the seat of the tap can become worn and irregular. These can be reseated to make them smooth and consistent, allowing for a water-tight interface between the seat and the washer. 

Mixer taps are generally more maintenance free than washer taps. When these misbehave we can replace the cartridge to make them work like new again.