Ultraman is the generic name for a series of Japanese live-action television shows produced between 1966 and 1998. Each series focused on at least one extraterrestrial "Ultra-hero" who arrives on Earth to fight various large monsters. Conveniently, each Ultra-hero can grow to over 200 feet tall. Each Ultra-hero had various energy emission powers, plus were experts in some form of hand-to-hand martial arts-style fighting. Virtually all of them also had one weakness: their energy would run out after a short time fighting. (Strangely enough, when this happened, they tended to "get serious" and use their most energy-draining attack on the monster(s).) When they weren't fighting monsters, they typically assumed the form of a human; each one either created their own human form, or merged with one (or, in one case, two) humans.
The Ultraman series are produced by Tsuburaya Productions. Tsuburaya Productions was started by Eiji Tsuburaya, the special effects master behind the early Godzilla films. His sons have carried on the tradition to this day.
The Ultraman concept began with a series concept called "Woo." The series featured a hero code-named Redman. This series concept was shelved when another series, Unbalance, was accepted. This series eventually became Ultra Q. The Woo/Redman concept, however, continued to be refined into the first Ultraman.
Tsuburaya Productions has created a common background for most of the Ultra-heroes, although a few (most notably Ultraman Scott, Ultrawoman Beth, and Ultraman Chuck) have not been directly connected to the Ultra-hero background. A second background has been given for Ultraman Tiga, but the source has not been confirmed. The following combines the latter conjectural background with the former official background.
Millions of years ago, at least one planet in Nebula M78 suffered a great disaster: its sun is destroyed. In the cataclysm, the human residents of the planet are transformed into beings of light. They discover that their new state offers the ability to protect the universe from the forces that threaten to recreate these events elsewhere, as well as other minor-by-comparison disasters. First, though, they create a plasma generator to substitute for their own sun. Their homeworld becomes the Land of Light.
(The following paragraph is derived from a Japanese BBS posting. While its "officialness" is in question, the story fits well.) 30 million Earth-years ago, at least four of these newly self-appointed guardians arrive on Earth, to fight various monsters threatening an ancient civilization here. They discover, though, that they cannot fight these monsters as mere beings of light. The three create bodies out of the Earth itself. These bodies are effectively battle-suits or war machines, motivated by the guardians of light. These were the first Ultra-warriors to appear in physical form. They protected the proto-civilization on Earth, until the planet was enslaved by Gidzera, a flower-beast whose pollen allowed it to control the minds of all who breathed it in. Gidzera caused the humans to reject the Ultra-Warriors. Perhaps by covenant, perhaps by some unknown physical law, the Ultra-Warriors were forced to leave, abandoning their bodies, leaving them as giant statues. One statue was hidden underground, along with that Ultra-Warrior's monster pet. (This statue would later become the "Evil Tiga.") Three others resided in a golden pyramid of light, normally hidden from human eyes, in the northern areas of Japan. One other statue resided elsewhere, until TPC found it & used it to create Man-made Ultraman Teranoid. Before they left, the Ultra-Guardians implanted a particular DNA signature in the genes of the humans; this signature would allow them or certain members of their descendents to transform into light and motivate the statues.
(The following paragraph has no basis in official information whatsoever. It is purely my own attempt at connecting the conjectural Ultraman Tiga background mentioned above with that of the rest of the Ultra-heroes.)
Learning of the need of physical bodies to further their work, the other potential Ultra-Warriors fashion their own bodies, perhaps from their home planets, perhaps from the planets they station themselves on. A universal police force is formed to organize these new warriors.
(Now we get back to the more official story.) Several of these new Ultra-warriors venture to Earth, specifically Ultraman and Ultraman Powered, to help protect the new race of humans that grew out of the ancient civilization. Other warriors moved to other planets, perhaps to help them overcome the same disaster that created the Land of Light. Areas known to contain other Ultra-Warriors include constellation U40, the planet Ultarra, and the Galactic Lion Cluster L77.