If we are friends on Facebook, which we likely are, since you are reading this, you may have noticed that for the last 36 days, I have been working on a picture album that is very near and dear to my nerdy heart. What I have done is compiled a miniature encyclopedia of the 35 Super Sentai of Japan. You can see it here. It covers everything from the first sentai, Himitsu Sentai Goranger, all the way up to new, anniversary series Kaizoku Sentai Gokaiger.

Goranger - Gokaiger
Unless you are as big of a nerd as I am, you are probably wondering "What is Super Sentai? Is it just Power Rangers?". I will answer the second question first, since it is the easiest. The short answer is no. The long answer is that Super Sentai, while stylistically similar to Power Rangers, is different in its tone and character/story depth. Let me give you an example:
In the original Power Rangers, the story was that the evil witch Rita Repulsa and her minions were sealed away by Zordon, because she is evil. She has no motivation; she is just evil. In the series it is based on, Kyoryuu Sentai Zyuranger, the evil witch Bandora was sealed away by the wizard Barza, when she tried to kill the dinosaurs, because a Tyrannosaurus inadvertently killed her son. This sent the once good woman into a rage. Here is another example from the same two series: In Power Rangers, the Green Ranger is a teenager who moves to town and becomes evil for a short time, when Rita places a spell on him. In Zyuranger, the green ranger is Burai, Geki's (red ranger) brother, who is resentful for being raised by their father's enemy. Rather than losing his powers like the green ranger, Burai dies.
In terms of demographic appeal, think of Super Sentai as more of a Simpsons than a Power Rangers (obviously not content-wise). It, like the Simpsons, appeals to people of most ages.
In order to answer the second question, allow me to give a brief history of Super Sentai. The first two Super Sentai were Himitsu Sentai Goranger and Dengekitai JAKQ. These were created by Shotaro Ishinomori, in 1975 and 1977. They were the origin of the team of super heroes aspect, that is in all following Super Sentai series. In 1978, Marvel Comics teamed-up with Toei Productions to create a live-action Spider-Man to air in Japan. For some weird (read: awesome) reason, Spider-Man piloted a giant robot, in this series. After this series, Marvel once again approached Toei to do a Captain America adaptation, with the same director, Saburo Yatsude. Things fell through and Yatsude ended up creating Battle Fever J, which is the first Super Sentai to have a giant robot. Essentially, Spider-Man and Marvel as a whole are responsible for Super Sentai AND Power Rangers existing in the form we know them in. Every series afterwards has kept the two, defining aspects of Super Sentai: a team of color-coded heroes and a giant robot.
I think sums it all up. I guess to end it, I will leave you with what amounts to the Alphabet Song of Super Sentai.