The terms speed and velocity are both used to measure the change in an object's motion. Although you might use speed measurements in daily life, physicists rely on velocity measurements more regularly. The major difference between these two terms is that speed doesn't include direction while velocity does.
Velocity is defined as the rate of change in an object's position over a given distance. The direction of this change can be either positive or negative, depending on the object's movement. This makes velocity a vector quantity, as it contains both magnitude and direction. The magnitude of an object's velocity depends on its displacement while the direction of its movement depends on its speed.
As a result, speed can never be zero while an object's velocity can be negative or positive. It's also important to note that an object can have both a positive and negative velocity simultaneously.
One way to understand this difference is to consider a running person traversing an athletics track. At any given moment her instantaneous speed will be the distance she covers during a complete lap of the track, but the average of her velocity will depend on whether or not she changes direction. If she runs around the entire track once, thereby changing her direction each time, then her average velocity will be 0.
