An endangered  species is one which has been categorized by the International Union for Conservation of Nature as likely to become extinct. Conservation biologists use the IUCN Red List, where "endangered" is the second most severe conservation status for wild populations, following critically endangered.
There are many species that are Endangered and one of the most important one is a tiger. 

Tigers are native to much of Asia, from some of the coldest regions to the steamy rainforests of the Indonesian Islands. They are the top predator in every ecosystem they inhabit.
Until the 20th Century there were nine tiger subspecies that probably numbered over 100,000 animals. They included the giant 660-pound, or 300 kilo, Siberian (Pantera tigris altaica) and Caspian (Pantera tigris virgata; now extinct) tigers as well as the relatively small—and now also extinct—200-pound (90 kilo) Balinese tiger. Depending on whether there are any remaining South China tigers—nobody has seen one in years—there are either 5 or 6 tiger subspecies remaining in existence; all are endangered. All tiger subspecies put together currently amount to around 3,200 endangered tigers remaining in the wild. 

TOP 10 MOST ENDANGERED SPECIES

1 Ivory-Billed Woodpecker

A North American bird so endangered it may actually be extinct

2 Amur Leopard

The world’s rarest cat: Only 40 left in Russia’s Far East

3 Javan Rhinoceros

No more than 60 of these swamp-dwelling Asian rhinos exist

4 Northern Sportive Lemur

Here’s the scarcest of Madagascar’s fast-dwindling lemur species

5 Northern Right Whale

Hunted to near extinction, 350 right whales still swim the Atlantic

6 The Saola—An Asian Unicorn

So rare it is almost mythological, the saola hangs on by its hoof tips in a forest full of poachers’ snares

7 Leatherback Sea Turtle

The population of the world’s largest turtle is dropping at an alarming rate

8 Siberian (or Amur) Tiger

The world’s biggest cat weighs as much as 300 kilos (660 pounds)

9 Chinese Giant Salamander

Humans are eating the world’s largest amphibian into extinction

10 The Little Dodo Bird

Samoa’s little dodo bird is in immanent danger of following the large dodo into extinction.

MOST CRITICAL SPECIES 

Kakapo Parrot

These flightless New Zealand birds are so rare they all have names

Greater Bamboo Lemur

Fewer than 100 of these Critically Endangered animals remain

Mountain Gorilla

Their habitat is shrinking, and fewer than 700 remain

Hawaiian Monk Seal

Scientists don’t know why this seal’s population keeps declining

Western Lowland Gorilla

Disease and illegal hunting are wiping out this gentle giant