In biology or ecology, a niche is a specialized area in which evolution optimizes certain species for. Some examples are predator, grazer, hunter-gatherer, scavenger, and parasite. Examples of animals occupying those niches are lion, deer, human, vulture, and roundworm. There are many different ways to determine and list these areas, including many possible levels of specificity.
The first niche was likely occupied by chemotrophs, microorganisms that gained energy from the environment by oxidizing inorganic compounds like sulfates. A couple billion years later, chemotrophs were followed by photosynthesizers, or producers/autotrophs, which gained their energy by using the sun's rays, carbon dioxide, and water to generate ATP, the energy currency of the cell. These organisms were cyanobacteria, and they persist in large numbers to this day. More modern autotrophs include trees, flowering plants, and vines.
For plants, the rainforest may be the best example of niche differentiation. There are plants that spend large amounts of energy in being taller than the others, and they make up what is called the emergent layer. They can spread out their branches and absorb light without competition, but must put so much energy into growth that the size of their foliage at the top is limited. Then there is the canopy layer, which seeks to be just average, taking in all the energy it can while competing with neighboring plants. Below the canopy are various specialized plants, such as vines and even carnivorous plants, like the pitcher plant. All these evolutionary strategies represent a distinct niche. Each one may be occupied by hundreds or thousands of competing organisms.
Among animals, the main categories are predators, consumers, and omnivores. There are unusual variants in these categories, such as scavengers and parasites. Ecologies tend to have a pyramid structure, with producers as the most organism, followed by consumers, then predators.