The Territory of a Population is the geographical place (ie continuous area) where this one find its vital resources. Populations can have overlapping Territories. A Territory has a maximal area: no species can control a very large zone.
Sometimes some individuals from a Population will leave the Territory to settle somewhere else (emigration phenomenon) or to pursuit a personal objective (curiosity, aggressiveness toward an enemy, etc; These are supposed to come back to their home Territory in this case.)
A population of Creatures (respectively plants) is a Creatures (plants) group living in the same Territory, belonging to the same specie and sharing almost all of their characteristics. All Creatures (plants) do not belong to a Population. Some Creatures (plants) are unique for example. A Population is a type of Community.