To name the many thousands of different plants, animals, microbes, and other living things on Earth, scientists use the Linnean (le-NAY-in) classification system. It was developed by a Swedish biologist named Carl Linnaeus (le-NAY-us) in the mid-1700s.

In the Linnean system, species are referred to by two Latin names. For example, human beings are classified as Homo sapiens. The first word refers to the organism's genus (Homo) and is called its generic name; the second word refers to the organism's species (sapiens) and is called its specific name. The generic name begins with a capital letter, but the specific name doesn't. Both names are usually written in italics.

A species is a set of organisms that can potentially mate and produce offspring. Members of a species tend to look more like each other, act more like each other, and share more genes with each other than they look like, act like, or share genes with members of other species.

Different species that share a large number of characteristics are gathered together into a group called a genus.

The Linnean system of classification is based on a hierarchy (or certain rank) and includes more than just genera (the plural of genus) and species. The hierarchy also includes families, orders, classes, phyla (called divisions if you're talking about plants), and kingdoms.

For example, genera that share characteristics make up a family. Families that share characteristics make up an order. Orders that share characteristics make up a class. Classes that share characteristics make up a phylum, or a division if you're talking about plants. And phyla (the plural of phylum) or divisions that share characteristics make up a kingdom.

There are five kingdoms in all: Protoctista (algae), Monera (bacteria), Fungi (mushrooms and mold), Plants, and Animals.

Some species, such as the fungus known as Trichoderma aggressivum, can be separated into even more specific groups, called forms. The green mold that attacked Pennsylvania mushrooms in the 1990s has been identified as having two forms: Trichoderma aggressivum f. aggressivum and Trichoderma aggressivum f. europaeum.

By Amy Spillman, Agricultural Research Service, Information Staff

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