A prairie refers to a plant community that is native to the ecoregion and is usually undisturbed by past farming and overgrazing from cattle. It can include rare plants that provide niches to wildlife, such as rare grassland birds.
A natural area may refer to a mixed plant community with native grasses as well as non-native plants like Johnson Grass or bushes and trees. Usually this is old farm or ranch land that still provides an important habitat for a wide range of wildlife.