Short answers, pulled from the story.
Cerberus is the multi-headed dog that guards the gates of the underworld in Greek mythology, preventing the dead from leaving. He was the offspring of the monsters Echidna and Typhon, and was most commonly described as having three heads, a serpent for a tail, and snakes protruding from his body.
Descriptions of Cerberus' heads vary widely across ancient sources. Hesiod gave him fifty heads, Pindar one hundred, and Horace a single dog head with one hundred snake heads. Later writers almost universally settled on three dog heads, which became the standard image.
According to Apollodorus, Hades agreed to let Heracles take Cerberus only if he mastered the dog without using his weapons. Heracles used his lion-skin as a shield, seized Cerberus around the neck, and squeezed until the dog submitted. Other accounts say Persephone simply gave Cerberus to Heracles in chains.
Yes. Apollodorus names the capture of Cerberus as the twelfth and final labour Eurystheus imposed on Heracles. Diodorus Siculus, however, places it as the eleventh labour, with stealing the Apples of the Hesperides as the twelfth.
The most widely cited entry point is Tainaron, a cape in southern Greece considered the most famous entrance to the underworld. Euripides, Seneca, and Apollodorus all place Heracles' descent there. Xenophon records a separate tradition locating the descent near Heraclea Pontica on the Black Sea.
The Kerberos computer-network authentication protocol, developed at the Massachusetts Institute of Technology in 1988, was named after Cerberus. Like its mythological namesake, it guards access to a network by preventing unauthorized entry.