According to International Union for Conservation of Nature and Natural Resources (IUCR) red list , the criteria for the classification of threatened specie and non-threatened specie is as follows:
1) Least concern: Ample population
These are the living organisms that are surviving on earth without any threat of extinction. They are scattered in different areas of globe, e.g Pines and Conifer trees.
2) Vulnerable: population of fewer than 10,000 mature individuals
According to Red list by IUCN, any specie of living organisms is called as vulnerable if its mature individuals are less than 10,000. They are the organisms which are hunted for trophies and pelts. For example, African elephants, Galapagos tortoises and Komodo dragons etc.
3) Endangered : population of fewer than 250 mature individuals
The specie that belong to this group face a great risk of extinction in very near future. Such organisms are being monitored by government conservation authorities and there is strict punishment on their harming or hunting. For example,blue whales,green sea turtles and Asian elephants.
4) Critically endangered: population of fewer than 50 mature individuals
The organisms in this group are likely to get extinct in near future if serious efforts are not made for their survival. The specie in this group are over hunted illegally, a process that is known as poaching. For example,California condor and mountain gorilla.
Hope it helps! :)