When the last individual of a species dies, that species no longer exists, and so becomes extinct.
Extinction can be a natural process caused by factors such as predation or disease.
But today, man has increased the rate of natural extinction by up to 1000 times, due to hunting, habitat destruction and global warming.
The IUCN - the International Union for Conservation of Nature - maintains a list of animal and plant species under threat, called the Red List.
The Red List is broken down into categories of threat, based on complex relationships between population size, and the area and quality of a species' habitat.
"Near threatened" means a species is likely to come under threat in the future.
Species currently in this category include the beluga whale, which has been most affected by overhunting.
"Vulnerable" means the population has declined by 50% within 10 years, or over three generations.
Species in this category include two great predators: the great white shark, a victim of trophy hunting, and the polar bear, whose habitat is being destroyed by global warming.
"Endangered" means a species is at high risk of becoming extinct in the wild.
Species at this level of threat include the snow leopard and the blue whale.
Population size will have decreased by 70% and the species will exist in no more than five locations.
"Critically endangered" means a species is at extremely high risk.
Population size will have decreased by 90% over 10 years or three generations, and the species will only exist in one location.
Critically endangered species include the black rhino, hunted for its horn, and the western gorilla, commercially hunted and made vulnerable by disease.
"Extinct in the wild" means no individuals of a species remain in their natural habitat but may exist in captivity, like the Barbary lion.
Extinct in the Wild
And "extinct" means the last individual of a species has died.
The IUCN Red List suggests that 40% of all species are currently in danger of becoming extinct.
And it is thought that tens or hundreds of species go extinct every single day.
This staggering rate of species loss suggests the planet is currently experiencing a sixth mass extinction event as the human population grows and our habitats expand at the expense of the natural world.