Speed is the measure of how fast you are moving.
Velocity indicates the speed as well as the direction in which you are moving.
e.g When your car’s speedometer shows that you’re moving at 40 km/hr, it is indicating your instantaneous speed, i.e. your speed at that instant. It means that if you continued to move at the speed you are moving now, you will be 40 kilometers away after an hour from the place you started.
But it doesn’t give you any information as to in what direction you are moving. You have to look at the GPS for the direction (or a map).
Hence, a statement “The boy is running at 10 km/h” is indicating his speed.
But the statement “The boy is running towards the bus at 10 km/h” is indicating his velocity. Here, the boy is moving in the direction of the bus, whatever direction that would be.
Speed and velocity are related to distance and displacement. The average speed over a time interval (duration of time) is the amount of distance covered in that time interval divided by that time interval.
e.g. if you take 40 minutes to reach your school, and it is 4 kilometers from your home, your average speed would be
( total distance covered)/total time taken = (4 km)/(40 minutes)
= 0.1 km/minute = (0.1 x 1000 m)/(40 x 60 seconds)
= 0.042 m/sec (approx)
It doesn’t mean that you were moving at the speed all the way, of course you stopped at traffic signals, slowed for a while to change directions and all. This is just the speed you were moving with, on an average on your journey to school. As you reduce the time interval, you get closer and closer to the actual instantaneous speed.