An egg is a vessel that contains an ovum — the reproductive cell produced by a female that would nurture and sustain an embryo. A chicken egg is the egg laid by a chicken. In general, eggs are composed of several parts: they have a hard outer shell, which would help protect a growing bird embryo (if the egg is fertilized); and inside the shell is the ovum. In a fertilized egg, a chick grows inside the egg for about 21 days before it is strong enough to break out of the shell and become a “fully-fledged” chicken.
In most mammals, ova that are unfertilized are periodically shed with the special lining of the uterus that would help nurture a growing fetus. Fertilized ova stay in place and become part of the building blocks that grows into an infant mammal. Chicken eggs are different in this respect, since hens lay both fertilized and unfertilized eggs, and most of the development of the chicken embryo in fertilized eggs occurs outside of the hen’s body. Special care of a fertilized egg is required for a chick to grow and hatch.
There are two main components of the ova in chicken eggs: the albumen or “white” of the egg and the vitellus or yolk. Most people will note some thick milky white membranous structures in the egg too. When not providing nutrition for growing chickens, these eggs are a source of nutrition for people and for animals that eat them. In fact, of all eggs consumed by people, those from chickens are most common, and recipes calling for eggs tend to mean ones from hens, unless they specify otherwise. Though there are specialty markets that sell emu, ostrich, quail, and duck eggs, these remain a tiny percentage of most eggs purchased by humans.
While the chicken was once hailed as perhaps a perfect food for humans, there are some concerns about consuming too many eggs. Though a single egg isn’t tremendously high in calories, the yolks do contain high levels of cholesterol. Some people avoid this issue by simply eating egg whites instead, which are high in protein, although not everyone enjoys eating the whites alone. There’s also some argument that sources of cholesterol in egg yolks may actually help lower overall bad cholesterol count, though this issue is debated.
Most people stick to eating the interior of the egg and either discard or compost the shells. Actually, eggshells are edible, and some people blend them into shakes. Eggshells may also constitute part of chicken feed since they have nutritive value. Different breeds of chickens lay eggs of different colors, but this has no direct impact on the taste or nutritional value of the egg itself.
One thing that may prove confusing for people is the different grades of chicken eggs and their different sizes. Eggs come in peewee, small, medium, large, extra-large, and jumbo in the US, although "large" is the most common size used. Moreover, some eggs are labeled organic and others may be sold as created by free-range chickens. The country in which the eggs are produced usually regulates the definitions of each of these terms and grades. Typically, free range and organic eggs will cost most, but the price is usually higher for larger eggs too. Some people purchase fertilized eggs because they’re lower in cholesterol.