Speed is the rate of change in position in a given amount of time. Thus, speed is measured in units of meters/second (m/s), kilometers per hour (km/hr) and more familiarly miles/hour (mph). Average speed is then the total distance traveled divided by the total time spent traveling.
Velocity has identical units to speed but the direction of motion must also be specified. In addition, velocity looks at the displacement (the shortest distance from the start of the motion to the end of the motion). This can lead to some interesting differences between average speed and average velocity.
Consider, for example, a runner on a 400 meter track. Suppose the runner completes exactly 4 laps in 6 minutes. Their average speed is then 1600 m/360 s or 4.44 m/s. However, there average velocity is zero! This is because their displacement is zero.
Another example would be if someone drives 300 m east, then 400 m north in a total time of 5 minutes. Their average speed is 700 m/300 s or 2.33 m/s. However, their displacement is 500 m from where they started. Therefore, their average velocity is 500 m/300 s or 1.67 m/s @ 53.13 degrees north of east.