Speed and velocity are both measurements of the rate of travel but their is a subtle difference.

Speed tells us the rate at which an object is moving.

Velocity tells us the rate at which an object is moving in a specific direction. 

Lets examine two simple scenarios:

1. A car moving forward at 5m/s

2. A car moving backward 5m/s

In scenario one the speed of the car is 5m/s and the velocity is also 5m/s.

In scenario two the speed of the car is 5m/s but the velocity is -5m/s.

We see that because the direction of the car is opposite in the second scenario we define its velocity as negative. 

More Information:

Speed and velocity are examples of a scalar and a vector respectively.

A scalar is any quantity which has only a magnitude and is not direction dependent.

A vector has both a magnitude and a direction.

Examples of Scalar Quantities: 

           distance, time, energy

Examples of Vector Quantities:

           displacement, acceleration, force