* Speed is a scalar quantity: Speed only tells us how fast something is moving, not its direction. It's always a positive value or zero.
* Velocity is a vector: Velocity, on the other hand, takes direction into account. So, a negative velocity simply means the particle is moving in the opposite direction of what we've defined as positive.
Example:
Imagine a car moving to the right. We could say its velocity is +20 m/s. If the car then starts moving to the left, its velocity becomes -20 m/s. However, its speed remains 20 m/s in both cases.
