An ecosystem is the community of living things from a particular area as well as everything else in the environment they interact with. So one example of an ecosystem would be a forest--it would include the trees and other plants, the insects that eat the leaves and the wood, squirrels that eat nuts, larger animals that the plant-eaters, and worms and fungi in the soil that decay dead things in the soil and release their nutrients. It also includes non-living things, like the energy the plants get from sunlight, the water and nutrients they get from the soil, and the air from which animals get oxygen and plants get carbon dioxide for photosynthesis. But you can also think of an ecosystem as being smaller or larger than this--you can think of a single tree and all the animals living on it as being one small ecosystem, all the trees on a particular hill as being a larger ecosystem, all the forests having a particular species of tree as being a very large ecosystem, and you can even think of the whole Earth as being one giant ecosystem. So I think that an ecosystem isn't so much a distinct "thing" out there as it is a way of looking at and thinking about the world--the point is that if you really want to understand the natural world, it isn't enough to just learn about all the different kinds of creatures that exist, you also have to understand all of the interactions that tie them together--how they all get the food and nutrients they need to survive, and who in turn depends on them for survival. Of course we are part of an ecosystem too, so if you want to understand what humans need to live, you also have to understand the whole ecological web that connects us to the other creatures living on the planet.