The story of Death Note is centered around Light Yagami, who discovers a notebook that allows him to kill someone just by writing a person's name in the notebook and picturing his or her face. Light uses his notebook to kill criminals in order to create a new, safer world. By possessing the Death Note, Light becomes known as "Kira", which is derived from the Japanese pronunciation of the world "killer."
While Kira has a number of followers, Light's attempt to create a new world is met by opposition, mainly the Japanese police force, L, Near, and the Special Provision for Kira, otherwise known as the SPK.
Anyone who has touched the Death Note is able to see a Shinigami, or god of death.