A crop is part of the digestive tract in many birds and some other creatures as well, such as insects and earthworms. Essentially, it acts as a storage container to hold food until it can be processed by the body. In some cases, people can even see the area on a bird; baby birds especially tend to have full, bulging crops that can look like strange cancerous growths in the bird's neck to the uninitiated.
Birds are probably the most famous for having crops, although the area works in much the same way in other animals. Essentially, since birds lack teeth, they rely on a pre-digestive system to process the foods they eat so that they can be broken down in the stomach. When a bird with a crop swallows food, it makes a stop in this part of the digestive tract, which softens the food before passing it to the gizzard, a muscular organ that mashes the food up, often with the assistance of small stones. Then, the food can be digested.
In addition to being used to prepare food for digesting, the crop also serves as a storage compartment, allowing birds to gorge. In scavengers, this can be extremely useful, as it allows the bird to glut on a good meal and process the food slowly. Other birds take advantage of it to eat a large amount of food and then dart to safety to digest it in peace, away from predators. The crop is also used to store food for young birds, with the parents regurgitating the contents on demand.
The crop is quite muscular, and it is designed to expand, sometimes to many times its size. When not in use, it can often look like a normal part of the esophagus, until it starts filling up with food, in which case it swells out like a little balloon. As food is pushed into the gizzard, the area shrinks again.
When the crop is full, it can often be felt. To feel one on a bird, a person can gently place a finger on the bird's upper chest, right around the area where the neck meets the chest. If he rubs the area, he will feel something which feels kind of like a sandbag, being very hard and obviously gritty. This is the crop, and if the person checks back in a few hours, it will probably have shrunk, unless the bird eats again.