Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th Ed.
A population is a group of individuals of the same species living in the same area. As such, a population is much like a city. Individuals interact with other members of its species within that populations. Although individuals may move to another area, therefor they may move to another population, this rarely occurs in nature.
Living in the same areas, individuals within a population will compete with each other for basic resources, such as living space, food. Individuals within a population will also compete for mates during their reproduction season.
Populations can be studied from several perspectives. Ecologist can look at a population's habitat, the size and growth of a population, a population's distribution and density, or a population's age or sex distribution. Ecologist can also study how individuals within a population interact with each other, and how they interact with their environment.