What is the difference b/w average velocity and instantaneous velocity?

Instantaneous velocity is the velocity at a particular instant.

 

Average is taken when the velocity is varying and then one can find average for the given time interval.

 

For example in a cricket match if one team score 200 runs in 20 overs , the average requried for the other team is 20 runs per over. It does not mean that the first team has scored 20 runs in each over, 20 is just the average.