A fief is an estate lorded over by a noble, who owns and oversees the land under the Crown. Tenants live on the fief and work for the noble. In turn, the nobles are supposed to look after their people, feed them in bad winters, and provide relief for natural catastrophes such as flooding.
The system is called feudalism and the nobles are called feudal lords.
In the Tortallan Universe
For a list of the fiefs of Tortall, click this link.
The feudalist system in Tortall operates with the idea of Noble Privilege. A noble cannot be touched by the Crown's Justice while in the comfort of his or her home fief.