A circular polariser allows you to block polarised light from any angle. While a linear polariser can only filter linear polarised light from one direction. A CPL offers more versatility and works better with digital cameras.
Both linear and circular polarizers look the same, and they both do the same thing – reduce reflections on non-metallic surfaces such as water, glass, and wet surfaces, and they increase color saturation too. A CPL filter is essentially a linear polariser with an extra glass element that allows you to adjust your lens filter to block polarised light from different directions.