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The family Ardeidae, with 72 recognized species, are referred to as egrets or bitterns or herons in the order Pelecaniformes.
Although herons resemble birds in some other families, such as the storks, ibises, spoonbills, and cranes, they differ from these in flying with their necks retracted, not outstretched.

Ardeidae, Ardea: the Latin word ardea meaning "heron".

The Pelecaniformes are an order of medium-sized and large waterbirds found worldwide.
The pelicans, shoebill and hamerkop form a clade within the order, with their next closest relatives being a clade containing the herons, ibises and spoonbills. 

Pelecaniformes    
  Threskiornithidae – ibises, spoonbills (36 species)
  Ardeidae – herons, bitterns (72 species)
  Balaenicipitidae – shoebill
  Scopidae – hamerkop
  Pelecanidae – pelicans (8 species)

The family Threskiornithidae, which belongs to the bird order Pelecaniformes, includes 36 species of large wading birds.