The three main categories of ecosystems are:
Terrestrial: Ecosystems found on land
Freshwater: Ecosystems not in the ocean that still hold water (with freshwater (of course))
Marine: Ecosystems completely covered/composed of deep salt water
Every animal in an ecosystem has a niche. What is a niche?
A niche is the role an organism plays in an ecosystem. Basically it is a job they do. Similar to chores you do at home.
In the ocean, the cleaner shrimp eats items off other creatures to feed itself and keeps the creatures healthy.
In forests, the beaver cuts down trees to make a dam so they can protect themselves, their young, and their food.
In many areas of the world, spiders make shells in their webs to protect their many eggs.