Leeches are annelids or segmented worms, which are classified as similar to earthworms. Although they are genetically similar, their behavior and anatomy are more specialized. They feed by clinging onto a host body and sucking their blood. Leeches mainly reside in fresh water lakes or swamps and their presence is usually undesired. Some species are also capable of living on land or in the ocean. Leeches have been used extensively in medicine for over 2000 years, and are still used to treat diseases such as gangrene even today.
Because they are well known as blood-sucking parasites, the term "leech" is often used colloquially to refer to someone who sponges off of others or off of the system.
Leech is also an archaic term for a doctor.