The Eight Gates are eight specific points on a person's Chakra Pathway System. They limit the overall flow of chakra within a person's body. The basis for the idea of the chakra gates comes from the body's limits on the functions within it. This makes the body much weaker, but it keeps the body from expiring too soon. By opening these gates, the user can surpass their own physical limits at the cost of extreme damage to their own bodies.
Rock Lee is able to open the first five gates. Might Guy, who taught Lee the skill, is able to open at least seven. Kakashi can open at least one gate, as shown during Kakashi's one-handed rock climbing where a shot of his brain shows the relevant point activating, as well as in his usage of the Front Lotus.
The state of having all Eight Gates open is known as "Eight Gates Released Formation" (八門遁甲の陣, Hachimon Tonkō no Jin; English TV "Eight Inner Gates Formation"). Any shinobi who activates this state will temporarily gain powers greater than a Kage, but they will die because of the damage done to their body. However, in Naruto Shippūden 2: Bonds, Shinnō managed to use a technique that allowed him to open all the gates without any drawbacks.
The Gate of Opening (開門, Kaimon), located in the brain. This gate removes the restraints of the brain on the muscles so 100% of their strength can be used whereas normally, a person can only use 20% of their muscles' strength to keep them from disintegrating. Unlocking this gate allows the user to use the Front Lotus.
The Gate of Healing (休門, Kyūmon; English TV "The Gate of Rest"), located in the brain. Forcibly increases one's physical strength and temporarily re-energizes the body.
The Gate of Life (生門, Seimon), located on the spinal cord. Allows the user to use the Reverse Lotus. The increased blood flow turns the skin red. In the video games, it allows the user to perform the Leaf Hot Wind.
The Gate of Pain (傷門, Shōmon), located on the spinal cord. Increases the user's speed and power. May cause muscle tissue to tear on use.
The Gate of Limit (杜門, Tomon; English TV "The Gate of Closing"), per above. Located in the abdomen.
The Gate of View (景門, Keimon; English TV "The Gate of Joy"), per above. Located in the stomach. Allows the user to perform the Morning Peacock. Opening of this gate releases such enormous amounts of chakra that it can cause nearby water bodies to form a vortex around the user.
The Gate of Wonder (驚門, Kyōmon; English TV "The Gate of Shock"), per above. Located below the stomach. Allows the user to perform the Daytime Tiger. Those who open this gate will pour glowing green sweat from every inch of their body, which immediately evaporates from their own heat energy, creating an aura which people would mistake as a green chakra coating. In the video games, allows the user to perform the Infinite Wild Dance. However, the side effects of opening the gate are that the user's muscle fibres are ripped to shreds, causing intense pain if anything or anyone touches him.
The Gate of Death (死門, Shimon), located at the heart. Releasing this gate uses up all of the body's energy. It makes the heart pump at maximum power and exceeds the power of every other gate. Opening this gate assures the user's death shortly after.