Echinoderm is derived from the Greek words for "spiny" (echino-) and "skin" (derma-). The "spiny" skin of these marine animals is displayed in most echinoderms and it is just one of a few other unique characteristics that include calcium plates, radial symmetry, and
a water vascular system. 
Under the superphylum Deuterostomia, the phylum Echinodermata is one of
four extant phyla of deuterostomes (distinguished from protostomes by the
development of the anus from the blastopore during embryonic development).*
The phylum Echinodermata can be divided into its five extant classes:*
Crinoidea (sea lilies and feather stars)
Ophiuroidea (basket stars and brittle stars)
Asteroidea (sea stars)
Echinoidea (sea urchins)
Holothuroidea (sea cucumbers)
*Phylogenetic Tree of Echinodermata below
All echinoderms are marine animals and approximately 7,000 living species of
echinoderms have been accounted for in the seas of the world today. 
Unique Characteristics of Echinoderms: , 
Calcium plates - these plates make up the internal skeleton of an echinoderm.
The plates are composed of calcium carbonate extracted from dissolved
calcium and carbonate ions in sea water. The plates can be rigid (sea urchins),
or flexible (sea stars and brittle stars) and provide a tough structure.
Radial symmetry - most echinoderms express pentamerism, a form of
radial symmetry. This pentaradial symmetry results in the development of a
system of fivefold symmetry - arms are developed in fives or multiples of five.
It is important to note, however, that pentaradial symmetry is secondary in
echinoderms and is only present after metamorphosis from the larval stage
(in which bilateral symmetry is expressed).
Water Vascular System - this system provides the mechanism for movement
in most echinoderms through a network of internal water-filled canals that in
many echinoderms form tube feet. These tube feet have "suckers" that allow
echinoderms to move or grip objects.