The Teutonic Order is a German holy order of knights which was founded in Acre. The Teutonic Knights were regarded as one of the most skilled military organizations in the world.
After the loss of Jerusalem the order was invited north by the Polish king's vassal - Konrad of Mazovia, to launch the Prussian Crusade. From East Prussia the Teutonic Knights campaigned against the pagan tribes and expanded around the Baltic sea beginning the conversion of the area to Christianity. The expansion of the Teutonic State soon became a threat to Poland who lost some of its territories to the Knights.Eventually the were beaten by Poland-Lithuania during XV-XVI century.
If you are playing as a Catholic ruler, you can hire them with piety, and when you're defending against infidels, you won't have any maintenance cost at all. But you should remember that they will only fight against heretics, infidels and rebel.