Railguns fire projectiles electromagnetically.
In science fiction and games, they are typically described as firing projectiles near the speed of light. Although if you calculate the amount of energy involved, that becomes extremely impractical as if the projectile weighed even one gram, the recoil on the gun would cause you to exceed the escape velocity of earths gravitational pull and when the projectile hit it would be on the order of a nuclear explosion.
At any rate, normal bullets routinely exceed the speed of sound so for a rail gun to be "cool" it has to be faster than that.