The World Conservation Union, or International Union for the Conservation of Nature (IUCN), Red List of Threatened Species is the world’s most comprehensive inventory of the global conservation status of plant and animal species.
The IUCN is the world’s main authority on the conservation status of species.
Species are classified in to nine categories – which are set through criteria such as rate of decline, population size, area of geographic distribution, and degree of population and distribution fragmentation.
The categories are…
- Extinct (EX)
- Extinct in the Wild (EW)
The term Threatened is a grouping of the three categories…
- Critically Endangered (CR)
- Endangered (EN)
- Vulnerable (VU)
Lower risk categories (which used to  also include a Conservation Dependant (CD) category) are…
- Near Threatened (NT)
- Least Concern (LC)
And these additional categories for species not yet categorised…
- Data Deficient (DD)
- Not Evaluated (NE)