Leeches are small slug-like creatures that live in water. They feed on human blood and/or that of other small invertebrates.
The Hirudo medicinalis species (vulgarly known as the medicinal leech) are used in potion-making: to make the Polyjuice Potion, four unsucculated leeches are required.  Also, a dash of leech juice is a required ingredient in the making of the Shrinking Solution.  Flabberghasted leeches are also used in potion-making.
Behind the scenes
- The use of leeches in the Polyjuice Potion was chosen by J. K. Rowling to represent "[sucking] the essence out of one and into the other". Interestingly, the film prop used for Moste Potente Potions suggests that the leeches used have their suckers removed.
- Harry Potter and the Chamber of Secrets (First appearance)
- Harry Potter and the Chamber of Secrets (film)
- Harry Potter and the Chamber of Secrets (video game)
- Harry Potter and the Prisoner of Azkaban
- Harry Potter and the Prisoner of Azkaban (video game) (GBA version only)
- The Road to Hogwarts Sweepstakes
- Wonderbook: Book of Potions