As the name suggests hard courts are made of uniformed, rigid materials with an acrylic surface layer. This surface is very popular across the world as it generally gives a good even and 'all round' playing experience and offers more consistency than other outdoor surfaces.
Although the speed of play on hard courts can depend on the exact material used, they are generally faster than clay, but not as fast as grass. Both the Australian and US opens are played on this surface. The US Open is played on an acrylic hard court while the Australian Open is played on a synthetic surface.