Serfs were sort of slaves who normally couldn't be sold away from the land. Putting it another way, they were tenant farmers who, in addition to paying a percentage of their crops as rent, owed a number of other obligations to their landlord. Among these are the duty to work on the landlord's private lands so many days a month and pay fees for things like getting permission to marry or for one's children to marry. Most important of all, the serf could not leave the lord's land without permission, making him one step up from a slave. While the serf had access to the courts, he could only seek justice in the manoral court (run by the landlord) not the king's court (which non-serf tenant farmers could use to sue the landlord in contract disputes). The Romans practiced a similar type of serfdom, and the German invaders had a form of it themselves. The Medieval version of serfdom was a combination of the two but was greatly weakened by changing economic conditions and the Black Death of the 14th century. Serdom still exists in some parts of the world.