A generalist species is able to thrive in a wide variety of environmental conditions, and if a heterotroph, has a varied diet. Specialist species can only thrive in a narrow range of environmental conditions and/or has a limited diet.

Omnivores, which eat both plants and meat, are usually generalists, but herbivores which eat a variety of plants may also be considered generalists. A well-known example of a specialist animal is the koala which subsists almost entirely on eucalyptus leaves. The raccoon is a generalist because it has a natural range that includes most of North and Central America, and it is omnivorous, eating berries, insects, eggs and small animals.

The distinction between generalists and specialists is not limited to animals. For example, some plants require a narrow range of temperatures, soil conditions and precipitation to survive while others can tolerate a broader range of conditions. A cactus could be considered a specialist species. It will die during winters at high latitudes or if it receives a lot of water.