There are three groups of worms. The simplest is the group know as flatworms (Platyhelminthes). They may be parasitic (flukes and tapeworms) or free living (planarians) in streams, ponds, and lakes. The second group is the roundworms (Nematoda) which are mostly parasitic of plants and animals. Many nematodes are found in soil, but are overlooked due to their small size. The third and most commonly seen group is segmented worms (Anelida) which includes earthworms and leeches. 

Roundworm    Segmented worm