In crop rotation a diversity of crops enters the field one after the other and over the years, this in contrast to mixed crops where multiple crops co-occur at the same time on the field. By using different crops, the risks are spread for the farmer, but it also has a positive influence on the soil. Because the crops change every (few) year(s), the development of soil pathogens is limited. Diverse rooting ensures a better development of the soil life. Finally we also see a reduction of the weed pressure.
Interesting links:
Crop rotation (Agricology) (English)
Scenarios crop rotation (LCV) (Dutch)