The International Union of the Conservation of Nature (IUCN) is a group that "helps the world find pragmatic solutions to our most pressing environment and development challenges". They are involved in supporting a variety of research programs, field stations, NGOs, and other environmentally driven projects. They strive to help communities and organizations implement the best policies and laws possible maximize sustainability.
The IUCN may be most famous for the creation of the "Red List" (or The IUCN Red List of Threatened Species). This list categorized the knowledge of and extent to which a species is in danger of extinction.
This categorization is ultimately brought down to seven categories for known animals. These are: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, and Least Concern. However, only Critically Endangered, Endangered, and Vulnerable species are considered the Red List.
There are specific classifications for each category that has been outlined by the IUCN. Extinct species are exactly what they appear to be: species that have gone extinct both in and out of the wild. "Extinct in the Wild" species have gone extinct in the wild, but are still alive in captivity and kept alive through breeding programs, etc… Critically Endangered species are those that are facing an extremely high likelihood of extinction in the wild. They are expected to decrease in population size by at least 80% within 3 generations. Endangered species are those which face a high risk of extinction in the near future. The IUCN estimates that approximately 40% of species on the planet are endangered. Vulnerable species are those which are at risk of extinction in the future, but not immeadiately. They are likely to become endangered if their conservation efforts are not improved. Near Threatened species are likely to become threatened in the future if effort to conserve them are not taken. This includes habitat protection and reproduction improvement. Least Concern species are those that do not qualify for the other categories. They do not have a high likelihood of endangerment or extinction in the near future and need to be monitored to ensure that they remain in this classification.