How does a population differ from a community?

A population is a group of a single species living in a certain area at a certain time.
A community is all combination of all of the populations in an area.
Populations are biotic parts of the environment because they are alive. When you combine all of the populations of say, a forest, (pine tree population, ant population, grass population, deer population, bear population) that group is called the community. When you consider both the biotic community and the abiotic factors in the environment (stuff like dirt and rocks and water and air) you have what ecologists call an ecosystem.