The Justice Society of America is the Golden Age version of the Justice League of America. A collection of heroes who banded together during World War II to help fight against enemies of all types and were called "mystery men" at the time because of their secret identities. The Justice Society would be the archetype of superheroes for generations to come and many of our best known heroes were from this Golden Age of comics.

Some of the earliest and most well known members of the original Justice Society: L to R - Wildcat, Jay Garrick, the Original Flash, Al Pratt, the Original Atom, Doctor Midnight, The Original Green Lantern, Hourman, Star Man and Hawkman.
The Justice Society of America, or JSA, is a fictional superhero team appearing in American comic books published by DC Comics. The Justice Society of America was conceived by editor Sheldon Mayer and writer Gardner Fox. The JSA first appeared in All Star Comics #3 (Winter 1940–1941), making it the first team of superheroes in comic books.

The DC Universe was once a collection of different companies whose properties were bought over time and as such were relegated to different Earths whose vibrational frequencies allowed them to exist at the same time on different versions of Earth.
DC would have many such Earths, where different companies heroes would continue their adventures under DC's banner. Earth-3 was filled with villainous versions of our favorite heroes, Earth-S held the Shazam family of superheroes.
These acquisitions would take place well into the eighties with the Charlton Heroes, (the Question, Captain Atom, Peacemaker and Judomaster) eventually getting their own Earth before the great Crisis on Infinite Earths tried to place them all on the same single world (unsuccessfully at first, I might add.)

We are introduced to the Justice Society in the seminal Flash of Two Worlds, where the Justice League discovers there was another Earth with different heroes who were a bit older, a bit greyer but were synonymous with the DC Universes' version of their prime Earth which at the time was dubbed Earth-1. The Justice Society would slowly be reintroduced to the DC Universe and would have several different names as they became more mainstream including the All-Star Squadron.
The team was initially popular, but in the late 1940s, the popularity of superhero comics waned, and the JSA's adventures ceased with issue #57 of the title (March 1951). JSA members remained absent from comics until ten years later, when the original Flash appeared alongside a new character by that name in The Flash #123 (September 1961).

Note the two Supermen. One is slightly older and greyer. He is the Earth-2 Superman, considered to be the very first version of the character later described as the Earth-1 Silver Age Superman, likely one of the most powerful versions of the character to have ever existed. Check out the different S-shields.
The Justice Society's characters (even though they were published first) were out of circulation and thus their Earth was dubbed Earth-2. For those with a bit of history, you know the introduction of the Earth-2 heroes spurred an event which happened every year after their revival where the Earth-1 and Earth-2 heroes would share an adventure, commonly dubbed the JLA/JSA crossovers. As a kid during that time, they were some of the most fun I ever had with comics. These crossovers also gave me an opportunity to discover an entire world of Golden Age heroes I never knew existed such as the Freedom Fighters and the All Winner's Squad.
During the Silver Age of Comic Books, DC Comics reinvented several Justice Society members and banded many of them together in the Justice League of America. The Justice Society was established as existing on "Earth-Two" and the Justice League on "Earth-One". This allowed for annual cross-dimensional team-ups of the teams between 1963 and 1985. New series, such as All-Star Squadron, Infinity, Inc. and a new All-Star Comics featured the JSA, their children and their heirs. These series explored the issues of aging, generational differences and contrasts between the Golden Age and subsequent eras.
While their star would never truly set, they would disappear from continuity for many years and for a time were actually killed off, presumably because they weren't bringing in the cash.
However, they would eventually get their own new title, Justice Society of America and it would with a reboot of the lineup, passing on of legacies and some of the best storytelling of the era, become a comic that lived up to the legendary nature of the characters and their histories.
The 1985 Crisis on Infinite Earths limited series, merged all of the company's various alternate realities into one, placing the JSA as World War II-era predecessors to the company's modern characters. A JSA series was published from 1999 to 2006. A new Justice Society of America series ran from 2007 to 2011. As part of DC Comics' "The New 52", an unnamed version of the team appears in the Earth 2 ongoing series (2012–2015) and Earth 2: Society (2015–).

One of the most interesting differences between the JSA and the JLA were the scaling up of their powers between the two eras of the Golden Age and the Silver Age. Silver Age heroes were larger and more powerful than their Golden Age counterparts.
Since the Golden Age heroes were essentially frozen in time, they experienced no progression of their powers and abilities. After the Crisis put them on the same Earth, there was a bit of a power creep to allow the Golden Age heroes (and villains) to hang with the often significantly more powerful heroes of the Silver Age.
One of the other differences was the passing of legacies to later generations. While most Silver Age heroes STILL haven't passed their legacy to the next generation, most of the Golden Age heroes had actually aged and had passed off some of their mantles to the next generation. Such heroes included: The Original Atom, Doctor Midnight, Mister Terrific, Hourman, Johnny Thunder, Starman and Wildcat. Batman and Superman had retired, passing their legacies off to Power Girl and the Huntress. (Yes, there was an Earth-Two Robin, but with continuity mashing after Crisis, he was retroactively erased...)

Justice League and Justice Society Pinup by George Perez.
JLA: Superman, Wonder Woman, Red Tornado, Batman, Martian Manhunter, Flash (Barry Allen), Zatanna, Hawkman and Hawkgirl, Firestorm (Ronnie Raymond), Atom (Ray Palmer) Aquaman, Green Arrow, Black Canary, Green Lantern (Hal Jordan) Snapper Carr, The Elongated Man.
JSA: Thunderbolt, Robin, Hourman, Wonder Woman, Superman, Starman, The Atom, Hawkman, The Huntress, The Flash (Jay Garrick), Doctor Fate, Johnny Thunder, Green Lantern (Alan Scott), Power Girl, Doctor Midnight, Wildcat.