Diet

Penguins are able to ingest food up to a quarter of their own body weight. The favourite diet of penguins is schooling fish (for example, Humboldt Penguins prey on anchovy). But penguins also feed on crabs and cephalopods like squid.
Antarctic penguin species prefer krill which is a collective term for different shrimp species that appear in big swarms.

Hunting

Penguins chase their prey as swift as an arrow. Abruptly, the head is stretched forwards and the bill snaps a schooling fish or krill. The upper part of the bill has a spiky hook which helps to catch the prey. Penguins have no teeth but spiky thorn-like structures on the tongue that lead the prey through the throat into the stomach.

Penguins always turn their prey so that the fish is swallowed head first. If they swallowed the fish tail first, the ventral or dorsal fin of the fish would get stuck in the throat of the penguin.

These photos show a Humboldt Penguin eating a fish in Tierpark Hagenbeck, Hamburg, Germany.