Cross contamination is what happens when bacteria from one food item are transferred to another food item, often by way of unwashed cutting boards or countertops, as well as knives and other kitchen tools, or even unwashed hands. Cross contamination can in turn lead to food poisoning.
Since dangerous bacteria are killed by cooking, the risk of cross contamination is highest where bacteria from a food item that needs to be cooked contaminates a food that doesn't need to be cooked. An example of this type of cross contamination is if a cook were to cut raw chicken on a cutting board and then later slice fresh tomatoes on the same board without washing it first.
Raw eggs (which like uncooked poultry are a source of the salmonella bacteria) are another common cause of cross contamination.
Preventing cross contamination requires good food safety habits such as frequent washing of hands, utensils, cutting boards and work surfaces. In professional kitchens, different colored cutting boards are used for different foods, such as raw poultry or fresh vegetables, so that cross contamination is less likely to occur.