The crop is like a storage area.
The gizzard is a muscle that processes the food...
Gizzard -
Muscular stomach. Its main function is the grinding of food, and partial digestion of proteins.
ultimatefowl .com/wiki/index.php?title=Gizzard
Gizzard -
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are also instrumental in this process. In certain insects and mollusks, the gizzard features chitinous plates or teeth.
Gizzard stones -
Some animals that lack teeth will swallow stones or grit to aid in digestion. All birds have gizzards, but not all will swallow stones or grit. The birds that do employ the following method of 'mastication':
"A bird swallows small bits of gravel that act as 'teeth' in the gizzard, breaking down hard food such as seeds and thus helping digestion." (Solomon et al., 2002).
These stones are called gizzard stones or gastroliths and are usually round and smooth from the polishing action in the animal's stomach. When too smooth to do their required work, they may be passed or regurgitated.
http://en.wikipedia.org/wiki/Gizzard
Chris