From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Kami (神, Kami; FUNimation "Guardian"; Literally meaning "God") is the title given to the individual protectors of planets. Despite the title, their role is closer to that of an overseer and they are not necessarily stronger than those whom they oversee (while Kami demonstrates that he is likely the strongest being on Earth at the time of his introduction during the Piccolo Jr. Saga, he is quickly surpassed by Son Goku and many others soon after). He remains Planet Earth's Kami, however, until the Imperfect Cell Saga over a decade later. Kami have an indirect reporting relationship to one of the Kaios (in the case of Planet Earth's Kami, this is North Kaio). Another way of looking at this is that Kami are more supervisors than they are managers. They have many of the powers associated with gods, including the ability to see and sense events around the world, to travel and guide souls to the Next World, and to be a general protector of the people. They also are long-lived, though not immortal. Unlike more traditional gods, the role of Kami is that of an official who can vacate and subsequently pick a mortal to succeed them. The average citizens of Planet Earth, however, are unaware of the existence of these Kami, as Mister Satan demonstrates by not understanding the role Dende plays.
Planet Earth had been overseen by a Kami for as long as the history given on Earth dated back. The Nameless Namekian (and Garlic in filler) assumed the role of apprentice to Earth's Kami in 431 Age. In 461 Age, the Nameless Namekian split into two and the good half assumed the role of Kami of Earth. This was the only way that the previous Kami would let him succeed him. The evil half renamed itself Piccolo Daimao and would go on to spark worldwide conflict. Kami and Piccolo Daimao were one and the same, and Kami was powerless to stop him due to the underlying fact that he would perish as well if he killed Piccolo Daimao. No other being on Earth, however, was anywhere near powerful enough to halt Piccolo Daimao's rampage. Luckily, after years of conflict, a martial arts master sealed Piccolo Daimao within a jar, trapping him inside for centuries.
Planet Earth saw a time of peace until Piccolo Daimao was unleashed on the world again over three-hundred years later, only this time he was stopped and killed by a new, far more powerful warrior on Earth, Son Goku. Goku soon after met and trained under Kami in order to stop Piccolo Daimao's son, Piccolo. After Piccolo was defeated in the Tenkaichi Tournament, relative peace reigned for five years. The arrival of the Saiyans sparked an entirely new conflict in which Kami trained Tenshinhan, Yamucha, Kuririn, Chaozu, and Yajirobe in order to combat the impending threat. Piccolo was later killed by Nappa in the battle and thus Kami died, leaving the Earth without a Kami for around a month until he was revived by the Dragon Balls on Planet Namek.
During the conflict against the Artificial Humans, Kami did little but offer support for the Z Warriors. He did evidently learn of Future Cell's arrival, however, thus opted to fuse with Piccolo and gain the original power held by the Nameless Namekian. This left Earth without a Kami for some time. Shortly after, Goku asked Dende, a young Namekian boy, to be Kami's successor and he accepted. Thus, after the defeat of Cell, a new Kami claimed the throne and created new, more powerful Dragon Balls... and a new era arose on Earth.
Seven years later, during the conflict against Majin Boo, Dende was nearly killed by Super Boo, but was saved by Mister Popo. Dende survived the remaining conflict and even traveled to the Sacred World of the Kais and watched Goku and Vegeta as they battled and defeated Kid Boo. After he returned to the newly restored Earth, a time of peace ensued.
By the time of Dragon Ball GT, Dende had become an adult and allowed Goku and his new trainee, Oob, to train in the Heavenly Realm. Dende provided support for the Z Warriors throughout the remainder of the series, as he was unable to do anything against the enormous powers of the villains who attacked Earth. He was taken under the control of Baby, but was turned back to normal with the use of the Sacred Water. After Goku defeated Omega Shen Long and departed with Shen Long, Dende returned to the Heavenly Realm and continued his role. Nothing beyond this was ever explored.
Planet Namek did not have a Kami because that was a term from Earth. It could be arguably assumed that the Kami of the Namekians is their Great Elder, Guru, who watched over his people much like Earth's Kami and who created a set of Dragon Balls for them. This was never officially stated.
After Guru's death during the Frieza Saga, Moori was given the position and control of the Dragon Balls (As soon as he was given the position, the Dragon Balls recharged immediately instead of waiting one Namekian year. As an added bonus, the Dragon Balls were now able to resurrect more than one being at a time, though those who died from sickness or of natural causes were still beyond their ability to resurrect).
In the anime series (not in the manga), North Kaio told Goku that Planet Vegeta was destroyed by the planet's Kami via a meteor storm he had summoned after he realized how evil the Saiyans were. Shortly afterward, it was revealed, however, that Freeza was to blame for Planet Vegeta's destruction. It is obvious that Freeza is not Planet Vegeta's Kami. It is possible that Planet Vegeta's Kami had acted behind the scenes to prompt the flash of paranoia that inspired Freeza's attack on the planet, but it is equally possible that Planet Vegeta's Kami simply did not exist. It is unclear whether North Kaio did not know the truth himself or decided to lie to Goku to prevent him from learning of Freeza. The second option is more likely considering that he later revealed that he was aware of the existence of Freeza and begged Goku and other characters to not fight him because of his immense power. It is more commonly accepted as an inconsistency created on the part of Toei Animation.
What appears to be the Kami of Planet Conuts appeared in Dragon Ball Z: Wrath of the Dragon.