In the series, ninjas use their "chakra," which is a combination of their physical and spiritual energy, in order to perform battle techniques.
There are five types of chakra: Normal, Biju, Celestial Gates, Healing and Cursed Seal.
Normal chakra is inherent to the ninja, while the Biju chakra emanates from a demon that has possessed a ninja's body. The Celestial Gates chakra allows access to all of the body's physical strength, while the Healing chakra is used by medical ninjas to heal injuries. Finally, the Cursed Seal chakra emanates from an evil symbol placed on the ninja's body.
Overuse of chakra results in death or a greatly reduced lifespan of a ninja.
Chakra in Naruto is very similar to "the force" in the Star Wars series, "chi" in the dragon ball series and magic points in RPGs such as Dungeons and Dragons.