The Ultra Series was made back in 1966 by the special effects studio Tsuburaya Productions, which was founded in 1963 by special effects wizard Eiji Tsuburaya, the man who brought monsters to life, like the King of the Monsters himself, Godzilla.
The Ultra Series began with the Twilight Zone inspired Ultra Q which featured mankind encountering the giant monsters known as Kaiju, but the franchise really took of with it's second entry; Ultraman the show, that introduced the first titular Ultra Crusader fighting of Kaiju, and alien invaders from space known as Seijin. Ultraman was pioneer in the beginning of Japan's "Monster Craze" so Tsuburaya's third entry into the Ultra Series; Ultraseven introduced the second Ultra Crusader, as well as expanding on the concept of space alien invaders.
Even after Eiji Tsuburaya's death on January 25 in 1970, his Ultra Series has continued to live on as one of his greatest creations.
We also recommend the YouTube channel Ultraman Explained, it is a small channel, but it is growing and it is one of the most resourceful placed to find Ultra Series informative videos. In fact they use this Wiki as one of their sources!
Please check out their videos, and if like their work maybe you could subscribe to their channel and help the Ultra Series community grow.