Crop rotation is the agricultural practice of cultivating a different crop on a given tract of land than was just harvested there. It is considered best practice to select a species of plant that is not closely related to the previous crop.
The advantages of crop rotation are:
Different plant species have different requirements for fertilizer and trace minerals. Rotating plant species or, ideally, even plant family, lessens the likelihood of creating nutrient deficiencies.
Plant pathogens are often fairly specific to a given species and may still be present in the soil. Growing an entirely different plant may force new pathogens to "start from scratch".
In some cases, the cultivation of one crop aids the soil preparation for the following crop. For example potatoes, which are harvested by digging them up, prepare the soil for carrots which require deep loose topsoil.
In a slightly different approach, even livestock may be incorporated into crop rotation. Farmer and rancher Joel Salatin, for example, incorporates cattle and poultry into intensive rotation on pasture. In this case the livestock are rotated rather than the perennial grasses they feed on. Demonstrable benefits for the livestock and the crop that they feed on (in this instance grass) are the result.