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This is a creature from Greek mythology, usually portrayed as the King of the Serpents. Its name comes from the Greek basiliskos, which means "Little King". It is very much akin to the Cockatrice, so much so that in some stories the two creatures are seen as interchangeable. They are different, however, although closely related. Stories disagree about how Basilisks are created: some state that they require an egg laid by a Rooster and incubated by a Toad, whilst others prefer a serpent's egg incubated by a Rooster. JKR goes with the first of these versions.
In the Harry Potter world, the Basilisk is an enormous snake, originally bred deliberately by the dark wizard Herpo the Foul. It is brilliant green (with the male also having a scarlet crest), can grow up to 50 feet in size, and can kill anything that looks into its eyes simply using its deadly gaze. It also carries lethal venom in its fangs and can kill by using this poison as well. Basilisks can live for hundreds of years and are uncontrollable by anyone except a Parselmouth.
Herpo created the first Basilisk after considerable experimentation revealed that using a Toad to incubate a chicken egg would produce the fearsome creature he was after. This very Basilisk was reputed to have lived for 900 years, although even this remarkable figure was outdone by the serpent from the Chamber of Secrets. This creature was placed in the Chamber by Salazar Slytherin and survived for 1000 years before it was unleashed upon the school twice during the 20th century, under the orders of Tom Riddle on both occasions. It killed Moaning Myrtle first time out, but met its match the second time, when a combination of Harry Potter and Fawkes the Phoenix defeated and killed it. It was showing no signs of old age at the time, however, and it can be assumed that it would have continued to live for considerably longer but for its untimely demise.
Spiders are known to particularly fear the Basilisk, and will flee if they sense the presence of one (although it is fair to say that most animals would also be wise to follow such a plan). The main weakness of the Basilisk is that the crowing of the Rooster is fatal to it. The breeding of Basilisks has been illegal since medieval times.
Basilisk venom is one of the few known substances that can destroy a Horcrux. The only other known one is the enchanted inferno Fiendfyre
