Short answers, pulled from the story.
Cadmus was the legendary Phoenician prince credited with founding the Greek city of Thebes. He was counted among the greatest heroes before Heracles, alongside Perseus and Bellerophon, and was credited by Herodotus with bringing the Phoenician alphabet to Greece.
After killing a water-dragon sacred to Ares at the Ismenian spring, Cadmus followed Athena's instruction to sow its teeth in the ground. Armed warriors called the Spartoi sprang up from the earth. Cadmus threw a stone among them, causing them to fight each other until only five survived. Those five helped him build the Cadmeia, the citadel of Thebes.
The Necklace of Harmonia was a bridal gift crafted by the god Hephaestus, given to Harmonia at her wedding to Cadmus. According to myth, it brought misfortune to every person who later possessed it.
Ancient tradition, recorded by Herodotus, held that Cadmus brought Phoenician letters with him from Tyre and adapted them for Greek use. Modern scholarship agrees the Greek alphabet derived from Phoenician script, though the timeline Greeks imagined was far earlier than the evidence supports. The earliest matching Greek inscriptions date to the late 9th or 8th centuries BC.
Ancient sources disagree. Herodotus and Euripides describe Cadmus as from Tyre, but Diodorus Siculus gave him Theban origins. Modern scholars have proposed Mycenaean, Cretan, and Argive hypotheses as alternatives. Some argue he was a fictional figure invented to explain the name of the Theban acropolis, the Cadmeia, and that his Phoenician identity reflected Greek perceptions of where the alphabet came from.
After years of misfortune attributed to his killing of the dragon sacred to Ares, Cadmus remarked bitterly that if the gods loved the life of a serpent so much, he might as well wish for it. He immediately began transforming. His wife Harmonia begged the gods to share his fate, and they granted her wish. In Euripides's The Bacchae, Dionysus prophesies that both Cadmus and Harmonia will become snakes for a period before eventually dwelling among the blessed.