A species is a group of genetically very similar animals, plants or other organisms that can reproduce naturally with each other and create fertile offspring.
The total number of species currently alive is unknown, and estimates vary widely. Some recent estimates place it at more than eight million, of which only about 14 percent have been described. The great majority of these are microorganisms, such as bacteria and fungi.
For example, there are more than 43,000 cataloged species of fungi, and estimates of the total number range from a mere 660,000 to as high as 5.1 million. Likewise, a single spoonful of soil might contain more than 10,000 species of bacteria, many of which have not yet been described, and each human body is home to hundreds or thousands of species of microbes. Moreover, all of these estimates are for currently extant species and thus are only a tiny fraction of all species that have ever existed.