Bullenbeisser are the progenitors of the modern Pit Bulls, Boxers, American Bulldogs, and a number of other breeds.
These dogs are properly thought of as a type, as there was a lot of variance.
Bullenbeisser were originally used as drover dogs for semi-wild pigs and cattle, and then found favor pulling small carts used for meat, milk, and bread sales.
One type of Bullenbeisser came from the town of Rottweil in southwesat Germany, and was known as the Rottweiler Metzgerhund or Rottweil butchers' dog. Today, a much changed (and not improved) version can be found in the Kennel Club, as the Rottweiler.