From Envirowiki

Eggs are half of the reproductive process of animals. When fertilised with semen, they produce new animals. Some eggs are fertilised and grow into animals internally in the female of the species (placental mammals, such as humans), some are fertilised internally and hatched externally (birds, monotremes), and some are both fertilised and hatched externally (fish, amphibians, etc.). Very few are fertilised externally and hatched internally (some frogs, sea horses). Eggs of some species are jelly-like, others are coated in a calcium-based shell.

The eggs of certain animals are used as food by humans, ie. chicken eggs, fish roe (or caviar). Eggs are high in protein, and have been shown to increase colesterol and cause heart disease.