Water bears are small invertebrates (0.05mm - 1.2mm long) in the phylum Tardigrada. There are over 900 species in this phylum, and members of Tardigrada can be found almost everywhere on Earth: from the frozen landscape of Antarctica to the hot, humid forests of Africa. Most Tardigrades are semi-aquatic, and you can probably find one of your own if you collect some lichen and have access to a microscope.
What makes water bears so incredible is their ability to survive in the harshest conditions known to man. Water bears have been frozen, dried, boiled, and even exposed to the vacuum of space. Through it all, water bears have survived by undergoing cryptobiosis: a state in which metabolic activity stops. It is like death, only reversible.