Speed is the magnitude of the velocity vector, and is therefore the scalar value corresponding to the velocity, regardless of direction (and always a positive quantity). "Speed" is a more colloquial term, since is ordinary conversation we are usually interested in how fast something is going, without having to qualify the direction. You can use Speed to calculate quantities where the vector nature of the velocity is not important – knowing the speed, for instance, you can calculate an objects kinetic energy (which depends upon the square of the magnitude of the velocity, but not its direction), but you could not calculate its momentum (in which the direction as well as the magnitude of the velocity are needed).