From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Ki is also known as "latent energy" or "fighting power". The term is the Japanese romanization of the Chinese term Qi (氣, Qi; Literally meaning "Breath"). This spiritual energy is a tangible internal energy inside a living being, its major focus being in the center of the body. By drawing it out, a person is able to manipulate and use it for performances outside the body. Ki can be used for many different techniques. Usually, the more concentrated the masses are, the more time the user requires to draw it out ("power up"). When a fighter gathers ki, they are able to gain enhanced strength, speed, and endurance, and can increase the power of their attacks to inflict greater damage to opponents.
Ki comes in a variety of colors. The most common color in the series is white. Other colors include yellow, red, purple, orange, blue, green, pink, and black. These colors vary on the technique being used, race that the character is, and how (im)pure the being's heart is.
The term ki was first introduced by Mutaito in a filler episode of the Dragon Ball anime when Son Goku traveled back in time and learned about ki. Although it is unknown who originally discovered this inner energy, the first usage of it was seen when Turtle Hermit performed an energy wave attack called the Kamehameha in order to extinguish the fire on Mt. Frypan. By drawing his ki into the palms of his hands, Turtle Hermit was able to expel an explosive mass of visible ki. This was the birth of energy wave attacks.
It is unknown whether or not incantations (such as "Ka... me... ha... me... ha!") must actually be spoken for the attack to be performed, or if the incantations were anything more than glorified battle cries. Often, the Z Warriors and their enemies performed attacks without saying anything, but there still existed the possibility that they were actually thinking the incantations rather than saying them aloud.
 Ki users
Main article: Saiyan
Saiyans and their hybrid counterparts were naturally adept at using ki. They generally had no difficulty manipulating their ki, and had even evolved the ability to increase it numerous times in battle without tiring easily. This was much unlike Humans, who usually spent themselves after the first increase. Unlike Humans and Namekians, however, Saiyans did not seem to have a natural ability to detect ki by sensing ki signatures. That ability had to be acquired through learning.
Main article: Namekian
Although Namekians were generally not as strong as Saiyans, their ability to use ki was as good as – if not more advanced than – that of Saiyans. Namekians used ki in more mystical ways than the Saiyans did, but used it in the same manner when fighting. Namekians also were able to naturally detect ki by sensing the concentration of it. Namekians also were able to sense the "evil" in a being's ki.
Main article: Human
It was very uncommon to see a Human use ki, though several Humans had been shown to acquire full mastery of the art. Among them were Yamucha, Tenshinhan, Kuririn, Chaozu, and Turtle Hermit. Another, lesser adept, Human was Son Videl, who was taught basic flight by Son Gohan (she could fly, but she was no match for the Z Warriors). Other users included Spopovich, Yamu, and Olibu (although Spopovich and Yamu were Majins and did not really count, and Olibu may have learned the art in the Next World). Humans also had a natural disposition to detect the concentration of ki that their opponents' possessed (though the ability had to be learned).
 Artificial Humans
Main article: Artificial Humans
Artificial Humans generally had an unnatural source of ki, therefore their ki capacity was unlimited. Most of the time, because it was unnatural, it was unable to be detected by sensing. Also, most of them were not able to detect ki by sensing. They had special devices to do that. Some Artificial Humans could absorb other forms of ki.
Main article: Kaio
A Kaio's ki was mostly invisible. Only two Kaios had ever been seen using it. Tthe first was King Kaio, who used his invisible ki to assist his pupils in his training. The second was Grand Kaio, who thrust his hand forward and caused what appeared to be a gust of wind to blow at Majin Boo, but it had no effect.
[[Auras] radiated from the user when increasing the concentration of ki inside their body. Warriors (excluding Artificial Humans) had a limit to the ki that their bodies could withstand, but the capacity for ki could be increased through mental and physical training. Some techniques (such as the Kaio-ken) rapidly increased the concentration of ki in the body, enabling the user to fight at a battle power that far exceededed their regular limits for a limited period of time. Piccolo Daimao was the first character shown to possess a visible aura.
Auras could also displace solid matter.
- Note: In the anime, auras were sometimes used for comical effect. For example, Son Chichi angrily producef a red-colored (Kaio-ken-like) aura after being angered by Maron (Kuririn's girlfriend) during the Garlic Jr. Saga. Although it appeared similar to the Kaio-ken, its red color was likely intended to comically represent her anger.
There were several Z Items in Dragon Ball Z: Budokai Tenkaichi 3 that changed the default color of a character's aura to a different one (for example, Aura Charge Red gave the character a red Kaio-ken-like aura) or granted an aura effect on characters (like Artificial Humans) who do did possess an aura.
- Note: In the anime, lightning auras were sometimes placed randomly on various characters regardless of their transformation or battle power.
Depending on their genetic inheritance, some beings were able to transform. Not all beings, however, were able to transform at will. In some instances, there had to be certain environmental conditions required for their transformation (like moonlight for Great Ape transformations) or emotional upheaval (for Super Saiyans). By transforming, a person acquired physical limits that somehow changed the ones of their normal body. Effects of transformations usually included alterations in strength and ki capacity. Some other effects included differences in speed and durability. The most commonly seen transformation was the Super Saiyan transformation, only accessible to certain Saiyans and Saiyan/Human hybrids. The negative aspect of most transformations was that the process itself consumed ki. The effect of ki loss when dealing with Super Saiyan transformations, however, could be limited through training.
Transformation was not always intended for increasing abilities. Some beings were known to transform in order to limit their abilities rather then increase them. An example of this was Freeza, who was first seen in a suppressed state, being that his fourth form was his original state, and the form that he was first seen in was actually a transformed one. Future Cell demonstrated a similar ability when he reverted to a larval state in order to fit into Future Trunks Brief's Time Machine and saved energy during the time that he had to wait for the Artificial Humans. Super Boo transformed into his original Kid Boo form after his counterpart was removed from his body.