A worm is an "elongated" invertebrate soft-bodied animal. The most widely known of all worms is the earthworm. The earthworm is a member of phylum Annelida. A side from that famous worm there are hundreds of thousands of different species of worms that live in a huge array of differet habitats.
Major worm categories include:

1. Acanthocephala - spiny headed worms
   
2. Annelida - segmented worms
   
3. Gnathostomulida - jaw worms
   
4. Chaetognatha - arrow worms
   
5. Nemertea - ribbonworms
   
6. Nematoda - roundworms
   
7. Nematomorpha - horsehair worms
   
8. Platyhelminthes - flatworms
   
9. Onychophora - velvet worms
   
10. Sipuncula - peanut worms

Throughout history worms have also beeen called helminths. This was especially true in zoological or medical terminology. Some insects such as larvae are often called worms. This Larvae include: the railroad worm, woodworm, glowworm, and the bloodworms.
Often times animals such as a dogs and cats, are sad to have worms. This means that the dog or cat is infested with parasitic worms. Parasitic worms are typically roundworms and tapeworms. Each worm species differes in it's abilities to move around on their own. Many worm species are bodied with no major muscles, thus preventing them from moving around on their own. These worms must be moved by external forces or other animals in their environment. On the opposite of these species there are several other species that have bodies with major muscles. These muscles allow them to move about on their own. These worms are a type of muscular hydrostat.