Grazing is a universal term in the retail business for the unauthorized consumption of a product that has not been properly purchased. It can be practiced by anyone from the soccer mom to the employee of the store. Eating a grape from the fruit section is already considered grazing, though it is unlikely anyone would be prosecuted for something of such a small magnitude.
More likely objects to be grazed are small snacks and candy bars, small drinks or other easily and quickly consumable objects. The grazer will simply make sure noone is within sight and hastily (or not, depending on how brash the grazer is) ingest the product. Store employees are the most likely grazers, but often do so with impunity, at least until a supervisor or store manager comes along.
Grazing is considered illegal and can easily lead to the prosecution of a customer or the discharge of an employee. To curtail the proliferation of employee grazing, many stores have made it mandatory to tape or otherwise affix the receipt to the item as proof of it's purchase.