Earth-Two was an alternate universe Earth that housed the Golden Age DC universe (the 40's and 50's), the Justice Society Infinity, and currently the Justice Society of America.
The development of Earth-Two largely mirrors that of Earth-One. It is divergent from the main Earth (Earth-One) in the period of time in which the heroes developed and operate, most of them originated in or around World War II. Its heroes experienced more or less parallel development with their Earth-One counterparts, with some exceptions. The heroes on Earth-Two occasionally interacted with those on Earth-One. Though Earth-Two bears much resemblance to the Earth-One universe, it does have some notable differences from Earth-One. The most prominent is the Earth's timeline, which is shifted back somewhat so that major events, such as the emergence of masked heroes and the explosion of Krypton, take place earlier in the chronology.
Earth-Two received much of the magic that had been shunted out of Earth-One, resulting in magical dominance over science in Earth-Two. Politically, Quebec is a separate nation from Canada; South Africa abolished apartheid earlier, in keeping with the shifted timeline; and Atlantis was raised from the sea and parts of it are ruled by a queen; as a result of being on the surface, Atlanteans have no underwater adaptations. The Legion of Super-Heroes does not exist in Earth-Two's future. The Justice Society of America and All-Star Squadron both are formed by a man named Franklin Delano Roosevelt. Earth-Two was destroyed during the Crisis on Infinite Earths.
Since Earth-Two is essentially parallel to Earth-One, it is home to many Golden Age (Golden Age -The era in comic books in which they became widespread and popular and when many comic book icons made their debut, beginning in the late 30's and ending around the mid-1950's.) counterparts of Earth-One heroes, including Kal-L, the counterpart of Superman; Princess Diana Trevor, the counterpart of Wonder Woman; Jay Garrick, the counterpart of Barry Allen; and Alan Scott, the counterpart of the Green Lanterns. Heroes native to Earth-Two who share no Earth-One counterpart, or who immigrated to Earth-One and became heroes there include Johnny Quick, Power Girl, and the Sandman.
Crisis on Infinite Earths (1985–1986) was an effort by DC Comics to clean up their continuity, resulting in the multiple universes combining into one. Since then, a handful of characters originating from Earth-Two have consistently remained part of the merged Earth, including Power Girl, Jay Garrick, and Alan Scott. Superman and Lois Lane from Earth-Two (along with Superboy from Earth Prime, and Alexander Luthor, Jr. from Earth-Three) were transported into a ghost-like "paradise dimension" tangential to the new universe.