* Speed is the *magnitude* of velocity. It tells you how fast something is moving.

* Velocity is a *vector*. It tells you both how fast something is moving *and* in what direction.

Therefore, if the speed stays the same, the velocity can still change if the direction of movement changes.

Example:

Imagine a car driving at 60 mph around a circular track. Its speed stays constant at 60 mph. However, its velocity is constantly changing because the direction of its motion is constantly changing.

In summary:

* Speed: How fast

* Velocity: How fast AND in what direction

To change velocity, you need to change either the speed or the direction, or both.