In statistics, a population is a set of things from which samples may be drawn. This allows statistical inferences to be drawn, or estimates made of the total population. For example, if we were interested in crows, then we would sample the set of crows which is of interest. The population of crows is limited to crows that exist now or in the future. Probably, geography will be a limitation in practice. To fairly represent the whole population, a sample needs to be random.
Population is also used to refer to a set of measurements or values, not only cases actually observed. Suppose, for example, we are interested in the set of all adult crows now alive in the county of Cambridgeshire, and we want to know the mean weight of these birds. For each bird in the population of crows there is a weight, and the set of these weights is called the population of weights.