Type species

The term type species is used in the scientific naming of living things, and is part of a system using a so-called "type".
The type system is central to the scientific naming of living things (organisms). How exactly it works is different depending on exactly what living thing is considered, but in a very general (and imprecise) sense it may be said that it fixes each name "to a particular specimen", the type specimen. This helps in finding out what particular name is to be used for a particular living thing (or a group of living things, a taxon). There are international Codes of nomenclature which set out exactly how types are to be designated and used.
For animals, the type species of a genus (or subgenus) is the type of that genus (or subgenus). The species (hopefully) has a specimen (or in some cases a group of specimens) as its type, so the type species is the link between a genus (or subgenus) and a type specimen.
For algae, fungi and plants, it is different: the type species of a genus is the species that has the same type (specimen or illustration) as the genus. For example, "Malus sylvestris", the European Wild Apple, is the type species of the genus "Malus". Thus, for algae, fungi and plants, a type species plays no great role; it is not an official term, but only a term of convenience.