The "population growth rate" is the rate at which the number of individuals in a population increases in a given time period as a fraction of the initial population. Specifically, population growth rate refers to the change in population over a unit time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period. (if you mean't population growth rate)
The population change refers to change in the number of people during the specific time. The world population has not been stable. It has increased manifold. This subject is very closely related to demographics, the statistics of how many people there are in a given population.