How velocity and speed are calculated
Average speed is total distance divided by total time:
v = d ÷ t
Velocity adds direction: it is displacement over time, and displacement is the straight line from start to finish. The two are identical for motion in a straight line without reversal, and they differ the moment someone turns around — a car that drives 10 km forward then 10 km back has travelled 20 km (speed 20 km over the trip) but has zero displacement and zero average velocity.
Worked example
A train covers 120 km in 1.5 hours: average speed = 120 ÷ 1.5 = 80 km/h. Scaled to per hour, that is 80 km/h — the same figure, just made explicit. If the train then reverses and returns to its origin, its displacement at the end is 0, so average velocity over the whole journey is 0, even though the speedometer read a healthy 80 the entire time. This is why "how fast was the car going" and "how fast was it travelling toward the office" can have very different answers.
Related ideas you will need next
Speed changes over time in almost all real motion, so acceleration is usually the more useful quantity — and with distance, velocity and acceleration you can reconstruct any constant-acceleration problem. For anything that moves through a medium rather than a vacuum, add the drag model: terminal velocity is where the driving force balances air resistance, which is why a parachutist stops accelerating.