Frankfurters are the ancestor of the ubiquitous hot dog sausage. Frankfurters are made with lean pork, which has been very finely ground, then filled into casings and smoked. American frankfurters contain both beef and pork. The Wiener or Vienna Sausage is a small "cocktail" frankfurter. Knackwurst is a small, plump lean-meat frankfurter containing both beef and pork.
Dowell, P., Bailey, A. (1980) The Book of Ingredients, Dorling Kindersley. ISBN 0718119150.