Prime Earth was formed by the temporal merging of Earth-0, Earth-13, and Earth-50 from the previous Multiverse, in an effort by a woman named Pandora and The Flash/Barry Allen to restore the Timestream to its intended nature.
In this newly made universe, costumed crime fighters are a relatively new phenomenon, with Superman being the first super-powered hero to come to public knowledge. Since then, several other heroes have come to the fore, with the formation of the Justice League. The success of the Justice League activities inspired the United Nations to create the government-sponsored team Justice League International.
Of course, there are also the supervillains, individuals that use their powers to commit evil, as well as N.O.W.H.E.R.E., an organization with a nefarious interest in superhumans and their abilities.