Population = all the freely interbreeding individuals of the same species living in the same place at the same time.
Community = all the populations of different organisms living and interacting in a particular place at a particular time.
Often, in a community, the population of one species in a given area depends on other populations of plants and animals for it's survival.
There is an animal community, and a plant community. So in woodland, oak trees would be a part of the plant community, and woodlice would be part of the animal community.
Dynamic (changing) feeding activity often occurs within a community, for example a caterpillar can only eat oak leaves during a certain time of the year.
Biotic component = an ecological factor making up part of the living environment of an organism eg. food availability, competition and preditation.
Abiotic component = an ecological factor that makes up part of the non-living environment of an organism eg. temperature, pH, rainfall and humidity.
An ecosystem is the interaction of the biotic components with abiotic components in a specific area.
Ecosystems vary in size - could talk about the ecosystem in a small pond, or the ecosystem of an area of ocean.
An ecosystem is pretty much self-contained in terms of energy flow and cycling of nutrients.... but if this becomes disrupted in any way the community is affected and the ecosystem becomes unstable.