The niche is the role of an organism in the community. An animal's niche is how it satisfies its need for food and protection. Zebras and lions have the same habitat, the grasslands of Africa. A zebra eats grass and a lion eats meat. Thus, they have different niches.
Any organism occupies a given niche.
The number of niches in an ecosystem is limited. In a certain area, two populations may try to occupy the same niche. Then they compete with one another. The one that is better adapted for the niche will win and will crowd out the other population. In a stable community, to organisms cannot occupy the same niche.