Samurai Jack's sword (a Japanese katana) was crafted by the gods Odin, Ra, and Vishnu, who took the good natured and indomitable spirit of Jack's father, Samurai Lord, and shaped it into a blade. Originally intended for Jack's father, he used it to defeat Aku when he first tried to take over the world. When Aku returned and captured him, he entrusted the sword to his wife to give to Jack when he came of age. Jack later claimed the sword, and it became his main weapon of choice, even when he was sent into the future.
Because of its god origin, it is extremely powerful, capable of cutting through anything (even adamantium, which was an armor used by Ultra-Robots). It is also consistently shown to be the only weapon capable of harming the nigh-omnipotent being Aku. It can only be wielded by one with a pure heart, as Aku once tried to use the blade and was unable to harm Jack.