In more detail
Average speed is total distance divided by total time; instantaneous speed is what a speedometer shows at a given moment. Velocity is speed with a direction, so a car going round a bend at a steady 50 km/h has constant speed but changing velocity. Useful conversions: divide km/h by 3.6 for m/s, divide km/h by 1.609344 for mph, and one knot is 1.852 km/h.
Why it matters
Average speed over a trip is not the average of the speeds along the way: slower stretches last longer and so count for more. That matters when estimating journey times or working out how fast you must go to make up lost time. Mixing units (km/h with miles, m/s with hours) is the other common source of wrong answers.
Example
100 km/h = 100 / 3.6 = 27.78 m/s
= 100 / 1.609344 = 62.14 mph
= 100 / 1.852 = 54.0 knots
60 km out at 60 km/h (1 h), back at 40 km/h (1.5 h):
120 km / 2.5 h = 48 km/h, not 50 km/h
Common confusion
Runners often use Running pace, time per distance, which is speed turned upside down: 12 km/h is a pace of 5:00 per km.