Short answers, pulled from the story.
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell, a zygote. Inside the vessel an embryo is incubated until it can survive on its own, at which point the animal hatches. In other kingdoms, similar reproductive structures are termed spores, seeds, or egg cells.
The largest recorded egg came from a whale shark and measured 30 by 14 by 9 centimeters. The ostrich egg, at 1.5 kilograms and up to 17.8 by 14 centimeters, is the largest egg of any living bird, while the bee hummingbird lays the smallest known bird egg at half of a gram, around 0.02 ounces.
Bird eggshells are made of calcium carbonate with a 5 percent organic matrix. The shell prevents the contents from drying out, limits mechanical damage, and protects against microbes while allowing gas exchange. It can range from paper thin up to 2.7 in ostriches and typically forms 11 of the egg's weight.
Egg color comes from pigments deposited over the white calcium carbonate base. Biliverdin and bilirubin give green or blue ground colors, while protoporphyrin IX produces reds and browns as ground color or spotting. Passerines mainly lay colored eggs, and protoporphyrin speckling also increases where soil calcium is low.
Platypuses and two genera of echidna, the spiny anteaters, are Australian monotremes and the only living order of egg-laying mammal. Their eggs are macrolecithal, very much like those of reptiles. Early extinct mammal species also laid eggs, which was probably the ancestral state.
Many vaccines for infectious diseases are produced in fertile chicken eggs. In 1931, Alice Miles Woodruff and Ernest William Goodpasture at Vanderbilt University found that the rickettsia and viruses behind many diseases will grow in chicken embryos. This enabled vaccines against influenza, chicken pox, smallpox, yellow fever, typhus, and Rocky mountain spotted fever.
Collecting the eggs of wild birds is now banned by many jurisdictions because the practice can threaten rare species. In the United Kingdom it is prohibited by the Protection of Birds Act 1954 and the Wildlife and Countryside Act 1981. Early collections have since come to museums, where scientists treat the collectors' notes as good natural-history data.