Velocity is a vector. It has magnitude and direction. If you try to describe a velocity, you can't do it with fewer than three quantities. "100mph, bearing 045 degrees, climbing at 10 degrees." It turns out that the simplest way to handles these three quantities is by resolving them into 3 component vectors in the way Student Pilot describes.
Speed is a scalar. It has only magnitude, no direction. For example, "90mph".
setVelocity is a buggy command and the scud missile itself is not a a proper object. You may run into all kinds of problems.