In biology, a species is a group of genetically similar organisms that interbreed. In addition, the offspring must be fertile; that is to say, the offspring must be able to reproduce as well. The classic example of two animals that can produce offspring, but are not in the same species, are the horse and the donkey. They can mate, and produce mules--offspring that are valued for their strength--but mules cannot reproduce. They are sterile, so horses and donkeys are considered different species.
The word species also refers to part of the scientific name that organisms have. Organisms are separated Domain, Kingdom, Phylum, Class, Order, Family, Genus and Species. Names are given by genus and species; genus is capitalized, and species is lower case. Most are from Latin and Greek words.
Finally, species are recognized by their chromosomal makeup. Typical individuals in the same species will have the same number of different chromosomes (haploid number) and ploidy (number of copies of each type). These chromosomes determine which genes are being coded for in the individual, even though different individuals may have different versions (alleles) of each gene.