An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters [Wikipedia 04 November 2010]
In biology and ecology, extinction is the end of an organism or of a group of organisms (taxon), normally a species. The moment of extinction is generally considered to be the death of the last individual of the group (although the capacity to breed and recover may have been lost before this point). [Wikipedia 04 November 2010]
Threatened species are any species (including animals, plants, fungi, etc.) which are vulnerable to endangerment in the near future. [Wikipedia 04 November 2010]
A Vulnerable species is one which has been categorised by the International Union for Conservation of Nature (IUCN) as likely to become Endangered unless the circumstances threatening its survival and reproduction improve.
Vulnerability is mainly caused by habitat loss or destruction. Vulnerable species are monitored and are becoming threatened. [Wikipedia 04 November 2010]
Critically Endangered is the highest risk category assigned by the IUCN Red List for wild species. Critically endangered means that a species' numbers have decreased, or will decrease, by 80% within three generations. [Wikipedia 04 November 2010]