Most of the feathers of the last 120 million years are/were of two types. Pennaceous feathers have a rachis (or vane) in the middle, while plumulaceous feathers do not. We know plumulaceous feathers as "down." Both are composed chiefly of barbs, which branch fractally into paired barbules. Pennaceous feathers use the barbules to make a watertight seal and reduce air friction, while in the tangle of hair-stuff that is a plumulaceous feather, trapped air provides insulation.
Pennaceous feathers subdivide two ways: contour and flight. In contour feathers the rachis is centered, contouring the feather to the body (picture the bottom of a boat). The rachis is off-center in flight feathers, which creates aerodynamic effects; these effects and their functions are determined by the relative volumes of barb on either side of the rachis. Examine the feathers from, say, an inner wing versus a wing-tip versus a tail for illustration. From small adjustments bloom large consequences.
Some feathers embody features of both pennaceous and plumulaceous feathers. These are known as "semi-plumes" and occupy the space between contour and downy feathers. Other, less common feathers include bristles and filoplumes, unbarbed sensory organs that register pressure and vibration. And eyelashes.
Feathers achieve color two ways: pigment, and splitting light.
There are three kinds of feather pigments: melanin, same granules of color that appear in human and animal hair; carotenoids, which are retained through the eating of pigmented plants; and porphyrins, which are derived from amino acids. These shine bright red under ultraviolet light, and appear on owls.
Peacock feathers are a good example of structural coloration--coloration achieved with microscopic surface structures. Structural coloration sounds sophisticated, but it likely originated during the Cambrian explosion 500 million years ago. Keratin is but one substance capable of it. Other common examples of structural coloration include hummingbirds, ducks, junebugs, and compact discs.
Keratinocytes achieve their purpose when they die. Their bodies, deposited en masse in a controlled way, become structure. The term for this is "integumentary appendage," and it also includes nails, hair, and scales.
The growth of feathers is insane.
Growth starts with a thickening of both the dermis and epidermis. This assembly of condensed cells is known as a placode. The placode becomes a feather germ after some elongation; the follicle forms by the proliferation of cells in a ring around the germ's base, with the old cells migrating downwards. This shortly creates a deep, ring-shaped depression around the feather germ. The portion of the germ surrounded by the depression is the follicle, which fills with dermal pulp and receives an artery. Continued downward movement of old cells from the follicle's epidermis forces the oldest cells up through the center. This is also how hair grows.
The rachis forms when, on the innermost surface of the follicle, ribs of keratin grow helically, meeting in a spine on one side of the follicle. The feather takes shape as the rachis emerges from the follicle and its ribs of keratin, its barbs lined with barbules, unfurl.
The first creatures known to possess flight feathers are carnivorous theropod dinosaurs. 125-million-year-old Microraptor gui fossils unearthed in China are fringed with the shadows of long, pennaceous, asymmetrical feathers; Caudipteryx fossils reveal feathers at the ends of unfused fingers. (Birds have fingers, but they're fused).
The origin of feathers touches the bigger question of evolutionary novelties--structures without antecedents or contemporaries. The current theory is that both kinds of feathers evolved in stages, beginning with a hollow cylinder similar to a pin feather. This evolved into a tuft of smooth barbs; this then evolved into a tuft of rough barbs, or a plumulaceous feather.
Pennaceous feathers began as smooth barbs fused symmetrically to a central rachis; they were not capable of generating lift. Next came barbules, then hooked barbules. Flight feathers appeared last. The creatures they clothed were hellish; Euornithes had a toothed beak as long as its body, and Archaeopteryx had a long, bony, feathered tail.
The aforementioned Microraptor had four wings.
The theory that feathers evolved in independent, novel stages is a recent one. The previous theory, toppled by the Chinese fossils, was that feathers evolved from elongate scales. Also diminished is the line between dinosaurs and birds. The functions of most early feathers is not completely understood; it is suspected they were used for courtship, defense, insulation, and water repellency.
Richard O. Prum and Alan H. Brush. "Which Came First, the Feather or the Bird?" Scientific American, 2014.
The Cornell Lab of Orinthology. "Feather Structure" and "Color."
Raptor Research Foundation. "Birds and Their Feathers."
The Scientist Magazine. "Color From Structure."