The fastest speed that can be reached is the speed of light. Nothing can exceed this speed according to Albert Einstein's Theory of Relativity.
Light travels slower in different mediums, just like sound.
For example, it travels faster in air than it does in water, but faster in a vacuum than in air. This is counter-intuitive in some cases. For example, you know that if you throw a baseball from a car in the direction the car is moving, then the speed of the ball is the speed you initially gave it plus the speed of the car. However, if you were to shine a spotlight forward, the speed of the light leaving the spotlight is the same whether the car is moving or not. In fact, you could shine the light in any direction with the car moving and it would still travel at the same speed.