OK, if wikipedia isn't good enough, here's the full Brittanica article
offspring of parents that differ in genetically determined traits. The parents may be of different species, genera, or (rarely) families. The term hybrid, therefore, has a wider application than the terms mongrel or crossbreed, which usually refer to animals or plants resulting from a cross between two races, breeds, strains, or varieties of the same species. There are many species hybrids in nature (in ducks, oaks, blackberries, etc.), and, although naturally occurring hybrids between two genera have been noted, most of these latter result from human intervention.
Because of basic biological incompatibilities, sterile hybrids (those incapable of producing living young) such as the mule (a hybrid between a jackass and a mare) commonly result from crosses between species. Some interspecific hybrids, however, are fertile and true breeding. These hybrids can be sources for the formation of new species. Many economically or aesthetically important cultivated plants (bananas, coffee, peanuts, dahlias, roses, bread wheats, alfalfa, etc.) have originated through natural hybridization or hybridization induced by chemical means, temperature changes, or irradiation.
The process of hybridization is important biologically because it increases the genetic variety (number of different gene combinations) within a species, which is necessary for evolution to occur. If climatic or habitat conditions change, individuals with certain combinations may be eliminated, but others with different combinations will survive. In this way, the appearance or behaviour of a species gradually may be altered. Such natural hybridization, which is widespread among certain species, makes the identification and enumeration of species very difficult.
