How is velocity different from speed?
Velocity has direction, whereas speed has only magnitude (size).
Speed is a scalar quantity so it only has magnitude (size).
Velocity is a vector quantity so it has magnitude and direction.
In practical terms this means that velocity is always measured with reference to a specific position. For example, it might be where the observer is standing. In that case a football moving away from the observer would have a positive velocity, but a football moving towards the observer would have a negative velocity.
In more complex situations an angle can be used to define the direction of the velocity vector. In those cases trigonometry and possibly the Pythagoras theorem are used for solutions.