Sentai is the Japanese show that Power Rangers derives from. Sentai has been running for about 20 years. The basic format of Sentai is a team of five, maybe six, heroes in spandex-clad suits, fighting against a person or organization usually termed as "evil", though sometimes that is not the correct way to describe it. In a typical episode, a monster is sent down, and the team meets up with it, and defeats it. The monster is then made big, and the team calls upon gigantic robotic-resembling fighters which combine into one big fighter. The giant robot destroys the monster. It sounds pretty simple, but it's not. Sentai lasts for only 50 or so episodes, after which the entire series ends. The next year, a new series begins, with an entirely different name and plot. This way, none of the characters ever get stale, and each year is a different one. This style usually means more closely integrated plots, since the length of the show is determined in advance. "Sentai" itself has no definite meaning, though its kanji symbols depict "war" and "troops". Many people tend to side towards "task force" or "battle squad". I myself would rather be a part of the side that chooses not to translate Sentai at all, but, in these guides, it feels awkward to speak of "Dinosaur Sentai Beast Rangers". So, I chose to define Sentai as "TaskForce", as most of the people on the Internet do.
It is hard to define when Sentai started. Some people say it started with Himitsu Sentai Goranger (Secret TaskForce Five Rangers). Others say it started with Battle Fever J, the first Sentai series made by Toei. Still others, though a minority, say that Sentai truly started with Battle Fever J's sequel, Denshi Sentai Denjiman (Electron TaskForce Electromagnetic Man). Regardless, Sentai has been around for a long time.
Sentai is actually a small part of something called "tokusatsu", meaning "special effects". Tokusastu extends to a wide variety of shows, some of which America is familiar with. Such shows include the legendary Godzilla and the universally-recognized Ultraman. Many of the other tokusatsu shows are incredible, towering over Sentai. Unfortunately, some of these shows have been converted into American shows like Sentai was converted into Power Rangers. For a glimpse of this, see below in Conversion Guide.
One final note: If at all possible, refrain from referring to "Japanese Power Rangers". I know that, if you have not been introduced to Sentai, it is only natural to say so, but do not say so. Many people, including myself, find the term offensive. It is akin to saying that Chinese looks like Japanese, or that the Batman comic book stems from the movies. It is wrong. If anything, refer to "American Sentai" (though even that may seem insulting to some, to consider Power Rangers a "Sentai").
Considering Battle Fever J to be the first Sentai series, Kyoryuu Sentai Zyuranger (Dinosaur TaskForce Beast Rangers) is the 14th Sentai series. This series aired in Japan in 1992. The series happened to catch the eye of Haim Saban, owner of a television company. He saw a new way to catch children's attention. Unfortunately, the Japanese actors would have to go. There was no way children would watch a show in Japanese or a show where the lips never match up with the voices. So, he decided to take only the scenes where the rangers were in their uniforms, and hire non-union actors to portray them out of costume. Fox Kids Network bought this idea.
The casting began. For some reason, Saban saw fit to replace Boi, the yellow TigerRanger, with a female. Most likely, he feared the feminists' responses to a team with a 5-to-1 or 6-to-1 ratio of males to females. If he only knew the jokes he would inadvertantly create :) The cast was incredibly diverse: a jock as Red Ranger, an African-American as Black Ranger, a Chinese girl as Yellow Ranger, a valley girl as Pink Ranger, and a brainiac as Blue Ranger. Saban evidently felt the pressure to keep a diverse cast, as every incarnation of Power Rangers from then on always had one black, one Asian, and (supposedly) one Hispanic on the team.
To keep the series adhered to the Zyuranger footage, the most outrageous and unbelieveable plots could pop up at any time. Much of the original dialogue was replaced with corny one-liners. Bandora, now renamed Rita Repulsa, constantly performed the same actions, as the producers desperately tried to keep her lips in sync with her voice, never really quite succeeding, and in the process giving her some of the worst lines and timing ever created.
Because of the despised format in which Power Rangers rips off the Sentai genre, it's questionable why anybody, anybody, who knows anything of Sentai would want anything to do with the show. Yet, almost everybody I've met on the Internet has a very good knowledge of the show, and many of them (including me!) watch the thing daily. Why? For one thing, the show has gotten better. Yes, I cringe at watching some of my favorite shows decimated, but some of the translations aren't as bad as they seem at first. A very precious few actually improve on the original. It's also very entertaining to see how the producers of the show manage to adapt some of the Sentai footage. For instance, in the PRT episode "Chase into Space", they adapted footage of imo-youkan flying into Exhaus' mouth by overlaying the yellow cubes with the Turbo R.A.M. And, for those with limited budgets, a slightly fulfilling dose of Sentai can be freely obtained by turning down the volume during PR-adapted Sentai fights.
In spite of the vulture-like behavior of Power Rangers pertaining to Sentai, it is still a fun show to watch. If you are into Power Rangers and Sentai, it is not something you "grow out of", like watching Barney or sucking a pacifier. This is something that stays with you the rest of your life.
Tokusatsu --- American Translation
Kagaku Sentai Dynaman --- Dynaman (USA comedy dub; 6 eps only)
Kyoryuu Sentai Zyuranger --- MMPR Season 1
Go Sei Sentai Dairanger --- MMPR Season 2
Ninja Sentai Kakuranger --- MMPR Season 3
Cho Riki Sentai Ohranger --- PR Zeo
Gekisou Sentai Carranger --- PR Turbo
Denji Sentai Megaranger --- PR in Space
Jikuu Senshi Spielban --- VR Troopers (J.B. and Kaitlin footage)
Cho Jinki Metalder --- VR Troopers (Ryan footage Season 1)
Uchuu Keiji Shaider --- VR Troopers (Ryan footage Season 2)
Denko Chojin Gridman --- Superhuman Samurai Cybersquad
Kamen Rider Black RX --- Masked Rider
Juukou B-Fighter --- Big Bad Beetleborgs
B-Fighter Kabuto --- Beetleborgs Metallix
B-Robo Kabutack --- Juuuust kidding there, folks, but I wouldn't put it past
Saban to try something like this.