Speed is a scalar quantity; that is, it can be described by a number only; for example 10 kilometers per hour. Velocity is described as a numerical value plus a direction. It is a vector quantitiy; meaning that it measures a total displacment of an object or person. If you were running and made a complete circuit; finishing from where you started, your velocity would be zero because you haven’t made any net gain in distance. Vector quantities, therefore have both a speed and a direction. You will increase your velocity as long as you keep moving in one direction. So that same runner, running the same distance in a straight line for 5 kilometers west at 10 kilometers per hour has travelled a total distance of .5 kilometers west.
In summary, speed is a numerical value only and velocity has a numerical value and a direction. Velocity = change in direction/change in time; whereas speed is the absolute value of distance/time. Absolute value means that if an object is moving forwards or backwards, it will not have a negative number in front of it. So if you travelled 30 kilometers per hour backwards from your current position, you would not describe your speed as -30 kilometes per hour; it would simply be 30 kilometers per hour.