All birds have feathers, and all birds fall into the Aves Class. The next level of organization is ORDER - what orders exist in the bird world?

Note that there do exist two sub-classes under Aves. These are:

Archaeornithes - distant bird ancestors, such as Arcaheopteryx
Neornithes - recent bird extinctions, such as the dodo

Now, on to the orders! The orders further categories the Aves class.

Anseriformes - ducks, swans, geese.
Apodiformes - hummingbirds, swifts.
Caprimulgiformes - nighthawks, whiporwills.
Charadriiformes - gulls, pipers, plovers.
Ciconiiformes - herons, storks, flamingos.
Coliiformes - mousebirds.
Columbiformes - doves, pigeons.
Coraciiformes - hornbills, kingfishers.
Cuculiformes - roadrunners, cuckoos.
Falconiformes - hawks, falcons, eagles, vultures.
Galliformes - turkeys, chickens, pheasants, partridges.
Gaviiformes - loons.
Gruiformes - coots, cranes.
Passeriformes - perching songbirds.
Pelecaniformes - pelicans, boobies, anhingas.
Piciformes - woodpeckers, toucans.
Podicipediformes - grebes.
Procellariiformes - albatrosses, petrels.
Psittaciformes - parakeets, parrots.
Pteroclidiformes - sandgrouse
Spheniscformes - penguins. Swimming flightless birds.
Strigiformes - owls.
Struthioniformes - emus, ostriches, kiwis, etc. Land flightless birds.
Tinamiformes - game fowl in south america.
Trogoniformes - trogons.

The Scientific Organization of Birds - the Actual Classification Tree
Scientific Definition of a Bird - General Overview
Bird Classification Example - Chickadee