Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called "speed".
That's the physics approach. Substitute 'velocity' with prędkość and 'speed' with szybkość, and you have the scientific explanation of the difference.
But in usual speech, they're usually synonymous. "Prędkość" rather than "szybkość" is used to describe the speed of movement, though.
Jellei, my wife is an evolutionary biologist, and she loved all the evolution sentences you (guys) made for this lesson when I showed them to her. For example, "the theory of evolution is very interesting". (Except that the picture for the word ewolucja actually shows DE-evolution, ha ha). But with the evolution sentences, and now this vector/scalar explanation, clearly you have a science background. No?
I wasn't among the people creating this course, so I'm not the one to take credit here :) And actually both the contributors that are responsible for most of the content that you see are students of English Philology, just like me. But I also study Transport, so that's a little bit of a technical background from my side ;)
Hmmm. Good question. I guess maybe here the scientifical accuracy is not the most important thing, it's enough to be correct in terms of everyday language. If we made 'velocity' one of the 'best answers', the number of people failing this exercise would rise significantly, especially among the non-native speakers of English.