Hard courts are often covered with an acrylic surface layer and have been historically used in the Australian and US Open since 1988 and 1978 respectively. Originally, tennis was played on hard courts, which where at the time made out of wood, they then lost popularity to the grass courts until recently.
Hard courts are, on average, slower than grass courts and are faster than clay courts. As you may remember, grass courts tend to get more sluggish over the course of the game, hard courts however actually tend to speed up. This is because the players actually wear down the surface of the court which in turn decreases the surface friction. The rebound velocity of the ball is comparable to that of the grass court but looks slower because hard courts have a higher coefficient of restitution which in turn makes the ball bounce back higher.