Penguins are birds, but they do not fly. One of the reasons is because their bones are heavy and solid giving them more weight for swimming and diving in water. Flying birds have light, hollow bones filled with air. The lighter weight of the bones make it easier for them to get off the ground to fly.
Penguins also have short stiff wings that act like paddles.
These flippers let them push through the water almost like they are flying through the air. Long, wide wings that flying birds have would be not be useful in the water. Penguins stand straight upright as compared to other birds that lean over. Their feet are far back on their bodies, which causes them to stand in an upright position. Penguins must stand straight upright because this is the only way they can balance their heavy plump bodies.