An ecosystem is a community of living organisms and all of the abiotic components or factors those organisms interact with, such as rocks, water, sunlight, and temperature.
Ecosystems are larger than a habitat, or a community or a forest type.
They also change. They change over time and they change in response to disturbances such as fires, the removal or addition of a species, or climate change. As an ecosystem forms, it will undergo what is called ecological succession. Eventually, if enough time passes, the ecosystem will reach what is called a climax community, when no new species are added and no species leave the community. This is when the ecosystem is considered stable.
There are many ways to define or classify an ecosystem. Using the dominant vegetation type is one method. The geology, climate, temperature and precipitation of the area are usually considered too. The geography is also often important.
There are multiple examples of ecosystems. You could consider the entire Arctic an ecosystem. Or you could be more specific and consider the boreal forests of North America. Even more specifically, one could consider the everglades a unique ecosystem.
There is no size requirement or limit to an ecosystem, although scientists typically refer to ecosystems on a rather large scale. Humans dictate the geographical constraints of an ecosystem.