Biological classification is a system used to organize living organisms into groups based on their shared characteristics. The term SPECIES is used to refer to the most basic unit of biological classification, which is a group of organisms that are able to interbreed and produce fertile offspring. GENUS is the next level of classification, which is a group of closely related SPECIES. Taxonomy is the science of classifying organisms, and GENERA are groups of related GENERA. Taxa are the highest level of classification, which is a group of related GENERA, SPECIES, and subspecies.