Chakra

Also see the original: Chakra

Chakra is a substance present in Shinobi; it is the signature biological element of Shinobi that sets them apart from normal people. Chakra flows through the Chakra System of a Shinobi, which acts similarly to the cardiovascular system. Along the Chakra System is a series of focal points known as Chakra Points. These mitigate and control Chakra flow throughout a Shinobi's body. Chakra is the catalyst that a Shinobi must use to perform Jutsu. When present outside of the body, it resembles blue fire. Chakra, in conjunction with Hand Signs, are molded together to execute the desired effect. Each Shinobi possess different levels of Chakra, however amount does not necessarily matter as much as being able to control and utilize Chakra effectively. Chakra is the life force of all Shinobi, and with each Jutsu that a Shinobi uses, Chakra is consumed. Chakra exhaustion can lead to extreme fatigue in a Shinobi, leaving them unable to continue physical activity, such as fighting. If a Shinobi would use all of their Chakra reserves, they would die. Symptoms of Chakra exhaustion include dizziness, lightheadedness, and passing out. If a Shinobi were to attempt a Jutsu that consumes more Chakra than they possess, the Jutsu will either not work or be a much weaker version than what is intended.

Chakra Nature
Each Shinobi has their own individual Chakra Nature. This can be either Fire, Water, Earth, Wind, Lighting, or Yin/Yang. Shinobi can only use Jutsu relating to their own Chakra Nature. Natures can be determined using Chakra Paper; these will determine a Shinobi's Primary Nature, however, over time a Shinobi may develop a second and third Nature as well, being able to utilize techniques in these areas.