Nematodes, also known as roundworms, are a diverse group of unsegmented worms belonging to the phylum Nematoda. They can be found in various environments, including soil, freshwater, and marine habitats. Nematodes play essential roles in ecosystems, such as decomposing organic matter and serving as food for other organisms. These tiny creatures vary in size, with some measuring just a few millimeters long, while others can grow to several meters. Nematodes can be beneficial, like those that help control pests in agriculture, or harmful, as some species are known to cause diseases in plants and animals, including humans.