The Thing is a very deadly extraterrestrial organism, which has the ability to imitate virtually any life form. It does not appear to have a default form, instead constantly swapping from one imitation to another. It can replicate any organism - plant, animal or human.
In order to replicate an organism, The Thing needs to be alone and in close proximity to its subject. Even a small part of it is enough to take over an entire organism. It will send tentacles out from its body to wrap and ensnare its victims. Using these tentacles it will then absorb the target organism, and slowly shape its cells to imitate them perfectly. Of course, this is very worrying as anybody could be The Thing, and it would be nearly impossible to tell merely from observation. This situation leads to trust being rapid eroded. One of the few ways to identify The Thing is by taking a blood serum test. Blood from the subject is mixed with with uncontaminated blood and heated. If there is an obvious reaction (the alien blood cells will attempt to escape the heat), you will know that the subject is not what they appear to be.
Very resilient in nature, every cell of The Thing appears to be able to operate on its own. Each part of The Thing is an organism in itself; every part is a whole, and every little piece is an animal. The creature's head may be severed, but it may simply grow legs and/or other appendages and act as a separate organism. Limbs, organs, eyes, mouths, etc can be formed at any point on The Thing's body.
The Thing is most obviously alien when it is in the process of absorbing and imitating another organism. During this transformation period, its body collapses into a random, mutagenic state. Skin and flesh bursts or rips open. Insectoid legs grow and tentacles lash out. Hair sheds as the bloody malformed body grows limbs, eyes, mouths, etc in weird, seemingly random places. However, after this period is over, The Thing will be a perfect imitation of its chosen target organism.
As a species, The Thing is very tough - cellular activity (potential life) can continue to linger in the remains of an imitated organism for a time even after its death. The Thing hides inside imitations, but one exposed as alien, it is vulnerable. Though it is strong even in the form of an imitated organism (it is powerful enough to force its fingers through human skin), it can be killed using fire and extreme temperatures.
An old organism, The Thing has a life span of 100,000 years at the very least. Presumably intelligent, The Thing is capable of space flight, traveling in a massive disk shaped flying saucer.