These animals have a segmented body, paired jointed appendages, bilateral symmetry, and an exoskeleton they shed periodically. This group includes trilobites, insects, scorpions, crabs, etc. Below are the more common groups.

Subphylums:
     Classes:

Trilobitamorpha
      Trilobita

Uniramia
            Myriapoda

Chelicerata
            Arachnida
                Meristomata

Crustacea
                 Malacostraca
            Ostracoda
          Cirripedia

For more information:

http://www.ucmp.berkeley.edu/arthropoda/arthropoda.html

http://www.palaeos.com/Invertebrates/Arthropods/8080C0Arthropoda.html