Annelida is a phylum of segmented worms that includes more than 17,000 known species found in a variety of habitats, including freshwater, marine, and terrestrial environments. They are characterized by the presence of repeating segments or rings, and many species have specialized structures such as bristles or parapodia that they use for movement and other functions. Annelids play an important role in many ecosystems, serving as decomposers, predators, prey, and hosts for various parasites. Examples of annelids include earthworms, leeches, and the marine bristle worms.