Trematodes are a class of parasitic flatworms belonging to the phylum Platyhelminthes. They are characterized by their leaf-like shape and are typically hermaphroditic, possessing both male and female reproductive organs. Trematodes have complex life cycles involving multiple hosts, including snails as intermediate hosts and vertebrates, including humans, as definitive hosts. Common examples include Schistosoma, Fasciola, and Clonorchis.