speed
Speed is how fast something is moving, plain and simple — the answer to 'how quickly are you covering ground?' A car doing 60 kilometres per hour, a walker doing about 5, a snail doing a fraction of one: these are speeds. Speed does not care which way you are heading, only how rapidly the distance is piling up.
Precisely, speed is a scalar equal to distance travelled divided by the time taken. Average speed = total distance / total time. If you want the speed at one instant (the number the speedometer shows right now), you take that ratio over a tiny time interval; that instantaneous speed is the size (magnitude) of the velocity, with the direction thrown away. Its SI unit is metres per second (m/s), though kilometres per hour (km/h) is common in everyday life.
The one thing to keep clear is the difference between speed and velocity. Speed is just a number with a unit (a scalar); velocity is speed plus a direction (a vector). A car going round a roundabout at a steady 30 km/h has constant speed but changing velocity, because its direction keeps turning. So 'constant speed' and 'constant velocity' are not the same claim.
You drive 150 km and it takes 2 hours, so your average speed is 150 / 2 = 75 km/h. This says nothing about which way you drove, and your instantaneous speed may have been 0 at red lights and 110 on the motorway.
Average speed spreads the whole trip evenly; the actual speed rises and falls along the way.
Average speed is total distance over total time, not the average of your two end speeds. On a trip where you speed up and slow down, those two ways of averaging usually give different numbers.