Leeches are in the same family as earthworms. Not all leeches are bloodsuckers, but the ones people are most familiar with are. They grow to around 2-5cm long but some species reach 30cm. Leeches live in damp, swampy areas.
Leeches have adapted one end of them into a sucker, so they can attach to a host. They inject anaesthetic in as they bite, so the host cannot feel them, and then an anti-clotting agent so the host's blood flows freely. Leeches stay stuck to their host until they are full, then they drop off.