How are species classified as extinct or endangered?

You may have heard different words being used to describe animals and plants whose populations are under threat like vulnerable, endangered or critically endangered.

But who decides how to classify these species into categories?

The International Union for Conservation of Nature (IUCN) is in charge of the Red List of Threatened Species which is based on how at risk a species is of extinction.

Dr Matilda Brown told Newsround all about it.