During the colonial era of New Spain during the 1500's, the Spanish crown granted Haciendas, or land holdings to the conquistadors and other nobles. This land was their to farm or to lease to other Spaniards of lesser social status. In comparison, the encomienda was not a land grant, but rather a system of labor, where the holder was granted the responsibility for a number of natives. Under this system, natives recieved protection from enemies and instruction in the Catholic faith in return for tribute. This was sometimes pain in the form of goods, but most often with labor.
This is where the mita comes into the picture. The Spanish colonizers borrowed the idea of required labor from the Inca Empire, who required its citizens to perform public service for a certain number of days out of the year. The remainder of the year was given to each man to farm his own land. While the mita system was considerably harsher under the Spanish conquistadors, it differs from slavery, which is defined as the forced ownership of human beings.