Form taxon

A form taxon is a group of organisms based on what they look like (their morphology). Such a taxon may not reflect their biological relationships. In other words, they may not be a clade. 
A good example are the terms snake and lizard, which are not good taxonomic terms because neither is a proper clade. On the other hand, all poisonous snakes and lizards are in a good clade, the Toxicofera.
Another good example are ducks. They are in the same family as swans and geese, but called "ducks" because of what they look like.