Speed most often describes acceleration or a high rate of motion. When something picks up too much speed, it can run off course, like a kid on a bike zooming uncontrollably down a steep hill.
Speed can be a noun or verb. As a noun, it's an indicator of pace, as in the speed of a car. As a noun, it's also a drug that keeps you awake, very awake. As a verb, it means to “move along quickly,” like how you speed around on your bike. A fun fact: while speed refers to the distance traveled by an object during a specific amount of time, velocity describes that distance, but in a specified direction.