Tardigrades (commonly known as water bears) form the phylum Tardigrada, part of the superphylum Ecdysozoa are microscopic, water-dwelling, segmented animals with eight legs.

The name Tardigrada means "slow walker". The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm.

Tardigrades occur over the entire world, from the high Himalayas (above 6,000 m), to the deep sea (below 4,000 m) and from the polar regions to the equator.
 

Tardigrades are able to survive in extreme environments that would kill almost any other animal.

Some can survive temperatures close to absolute zero, 1,000 times more radiation than any other animal, nearly a decade without water, and even the vacuum of space.

Description source: Wikipedia - The free encyclopedia