<------ Chakra Network System
To perform techniques, ninja need to use their chakra. Chakra comes from two places: (1) the body energy inherent in the trillions of their cells and (2) the mental & spiritual energy gained from exercise and experience. To perform a jutsu, the ninja will bring out and release these two energies. The amount of chakra required for a technique will vary depending upon its size and complexity. Each ninja has the potential to better utilize an elemental Ninjutsu style. For more on elemental chakra and jutsu, click here. For Taijutsu the ninja does not need to necessarily use chakra, as some attacks merely use the stamina required to carry out the attack. For Ninjutsu the ninja's chakra control will determine how much chakra is used on the technique. For ninja with excellent chakra control, they will use the minimum amount with nothing wasted. For ninja with poor chakra control, they will expend more chakra than needed to accomplish their jutsu.
Chakra flows through the body's chakra circulatory system. On a more microscopic level is these channels connecting all the vital points on the body. These small channel pathways travel between the cells of the body. They carry the generated chakra and allow the ninja to mold it. If these channels are severed, it is beyond the ability of even skilled medical ninja to repair. On the large scale the chakra circulatory system travels throughout the body and around the internal organs also responsible for creating chakra. This system includes 361 tenketsu, which are tiny pin points through which the chakra flows. The chakra flow can be increased or decreased by manipulating these tenketsu. The chakra circulatory system is only visible through the use of the Byakugan. The Sharingan can see general chakra flow, but not the internal system itself. The Byakugan and the Hyuuga clan's Juuken (Gentle Fist) attack style allows them to eject their chakra from their hands into the tenketsu of their opponents. This pressure will cause the tenketsu to close and block the flow of chakra. With enough tenketsu closed, the ninja's chakra flow will be almost nothing, eliminating their ability to use jutsu. Also because the system travels throughout the body and around the internal organs, pressure done against the tenketsu will also damage nearby organs. Because a ninja can not train and strengthen their internal organs, such attacks can be devastating.
Focusing an equal and constant amount of chakra to a portion of the ninja's body allows them to attach their body to the touched object. This would allow them to walk up vertical surfaces. By expelling a continuous set amount from their feet in proportion to their body weight, the ninja can walk on water. Ninja can also focus chakra to specific limbs, which will increase the muscles' power and strength.
Chakra can also be extended from the body for attack and defense. This can take the form of the chakra strings used to manipulate puppets in Kugutsu no Jutsu. It can also be focused to a fine point for cutting, such as in Kabuto's chakra scalpel. It can also be used to form a defensive barrier in Chakra no Tate.
The chakra in the brain and sensory organs are also the primary target of Genjutsu. Genjutsu is created when a ninja extends their chakra flow through the cerebral nervous system of their opponent to control their minds chakra, thereby affecting their five senses. To combat the effects of Genjutsu, there are two options. The first option is for the ninja to stop the flow of chakra in their body, and then apply an even stronger power to disrupt the flow of the casters chakra. The second option is to have an outside ninja unaffected by the Genjutsu make body contact, and use their chakra to disrupt the casters flow.
The Eight Gates
In addition to the tenketsu, the chakra circulatory system houses eight valves responsible for regulating the flow of chakra. The Hachimon or eight gates are located along the spine, the heart and in the ninja's head. It is possible to force the body to open these gates, which allows the ninja to release ten times the usual amount of power. Unfortunately such power comes at a price, as each subsequent gate is opened, the ninja increases the damage to their body and comes closer to death. For this reason gate opening has been deemed Kinjutsu. If all eight gates are opened, the ninja gains power that surpasses the Hokage at the cost of their own life. Also even if the ninja opens fewer gates, if they do not strike a finishing blow against their opponent, they will be severely weakened and very defenseless to a counter attack. It is also unclear whether the ninja has to open all the gates in order, of if they can open a later gate and bypass the previous ones.
The Eight Gates Are;
 Kaimon (Opening Gate)
 Kyuumon (Rest Gate)
 Seimon (Life Gate)
 Shoumon (Wound Gate)
 Tomon (Limit Gate)
 Keimon (View Gate)
 Kyoumon (Wonder Gate)
 Shimon (Death Gate)
training (practice makes perfect :P)