What is Kamen Rider and why you should watch it?
Kamen Rider is a Japanese series of the genre Tokusatsu, one of the most beloved franchises in Japan and one of the most profitable properties for Toei and Bandai. Created in 1971 by Shotaro Ishinomori (Creator of Cyborg 009 and student of Osamu Tezuka) originally thought as an adaptation of his Skull Man manga, but after being advised by his producers, he changed the skeletal hero for one themed by an insect, the Grasshopper, that would become a common theme for other members of the franchise.

The original series focuses on Takeshi Hongo (played by Hiroshi "Segata Sanshiro" Fujioka) who not only acted in the leading role but performed all the fights within the suit and motorcycle stunts. Due to this dedication he ended up hurt and had to be replaced by another actor who became Kamen Rider 2. Since then each new series (except for Kamen Rider Black and Black RX) had a new protagonist and a new story.

Kamen Rider can be divided into 3 Eras. The Showa Era, from 1971 to 1999, where all the heroes (or the great majority) are paragons of good and justice, they are usually cyborgs and they all fight against different incarnations of the same secret organization or similar groups. The Heisei Era from 2000 to 2009 covers 10 series that rediscovered and altered the foundations of the franchise for the new millennium and finally the Neo Heisei Era, from 2010 to the present with the newest series that have a kind of continuity.

The charm of Kamen Rider is that there is possibly a series for every taste, as each season has new enemies, new themes and new protagonists. As a rule, Kamen Rider tends to target a teenage audience (Shonen) unlike the Super Sentai (The series that serves as the basis for Power Rangers) that targets a younger audience. If you want a series with elements of Terror and Police you have Kamen Rider Kuuga, you want one centered in Japanese mythology and you have Hibiki, you want science fiction and you can see Kabuto.

The series of the Showa period are usually very similar to each other (with notable exceptions). 
You want to read a Spanish version of what I've said? Here you go!
https://kumacorner.wordpress.com/2018/11/27/por-que-deberias-ver-kamen-rider/