Velocity has a DIRECTION as well as a MAGNITUDE (that is, velocity is a VECTOR quantity if you've learnt that term yet)
The magnitude of velocity is called SPEED (just a POSITIVE number/scalar, not a vector.
Every work day, I leave my home to drive to my work which is 30 Km west . When I am on the highway I set my cruise control to 120km/hr. That is my speed it is the same both on my way to work and on my way home. My velocity however is different in the morning then in the evening. In the morning my velocity is 120 km/h WEST, in the evening it is 120km/h EAST. Thus it requires 2 numbers to specify velocity, it is possible to have a negative velocity. Speed on the other hand is a single positive number.