As mentioned before, the name "Tardigrade" means slow-walker in Latin. Which is convenient, because they are, indeed, slow walkers. 

     The phyla tardigrada is composed of over 700 known species. I outlined 2/3rds of the phylogenetic tree here, just to give you a rough idea of the enormous number of species. However, remember: this is not complete:

http://i183.photobucket.com/albums/x86/Icelandic1717/Eutardigrada.jpg

     Also as I mentioned in the introduction, the water bears are related to the phyla Arthropoda--meaning at one point in the developmental history, both of these phyla probably had a common ancestor.

Characteristics of the phyla Tardigrada, via Earthlife.net:

             Characteristics of the Tardigrada:
1) Bilaterally symmetrical.
2) Body has more than two cell layers, tissues and organs.
3) Body cavity is partially a coelom.
4) Most possesses a through straight gut with an anus.
5) Body monomeric.
6) Body possesses 4 pairs of unjointed claw bearing legs.
7) Body possesses a fixed number of cells (eutelic).
8) Has no circulatory or respiratory system.
9) Primitive excretory organs in some species.
10) Reproduction normally sexual and gonochoristic, but can be parthenogenetic.
11) Feed on a plants or small animals.
12) All live in aquatic or damp places, normally associated with vegetation.