All penguins are black with white undersides, and are commonly described as wearing tuxedos. This color pattern acts as camouflage when the penguin is swimming, protecting it from predators. From underneath, the white belly blends with the bright water surface, and fromabove, the black back is indistinguishable from the dark water. 

Penguin species can be grouped according to common characteristics. Banded penguins have black and white stripe patterns on their chests and heads. The crested penguins all have bright yellow or orange plumes on their heads. Brushtail penguins have long stiff tail feathers. The king and emperor penguins have bright yellow and orange chest and head patches, and the yellow-eyed penguin has a yellow crown. The fairy penguin's feathers are bluish. The emperor penguin is the largest, at nearly four feet tall and seventy-five pounds. 

The small fairy penguin is sixteen inches tall and about three pounds. All have solid, heavy bones that help them dive deeply into the water. They have streamlined bodies that move smoothly through the water as they pump their strong, flipperlike wings and steer using their webbed feet and tails as rudders. Penguins can hold their breath for many minutes at a time, and they frequently leap out of the water, porpoiselike, to take inmore air. On land, penguins walk with an awkward sideways waddle. Because their short legs are set back on their bodies, they stand erect and must hold out their flippers for balance. Penguins often toboggan themselves by flopping on their bellies and pushing with their flippers and feet. Penguin feathers are tiny and stiff, overlapping to form a waterproof coat. 

An underneath layer of down helps to trap warm air and protect the penguin from the cold water and wind. Penguins of the Antarctic region have an insulating layer of blubber. Those in temperate climates often have to cool themselves down by ruffling their feathers and holding out their flippers. They can control the flow of blood to their unfeathered areas, such as the feet and under their flippers, which helps regulate their body temperature. Penguins preen their feathers regularly, to spread waterproofing oil from a gland near the tail.

Penguins have very short legs, which are used under water for steering. On land it's not easy for a penguin to walk. Waddling seems to be the easiest way to walk with short legs (in fact this has been researched by scientists). In addition, some species displace themselves by hopping over the ground.

Although you don't see them, penguins do have knees and upper legs. They are completely enclosed within the penguin body. The upper leg is oriented horizontally. In contrast to other birds (whose feet are placed in the middle of their body) penguin feet are set all the way back, causing an upright stand.

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