Earth-2 was the first of the so-called "infinite Earths" discovered by the inhabitants of Earth-1, where the major super-heroes and villains originated in the 1930s and 1940s and battled in the Second World War.
The heroes active by the present day were either elder heroes from the past or their children. This universe was notable for containing Keystone City in place of Earth-1's Central City and the Daily Star in place of the Daily Planet. It also had the original, older versions of Superman, Batman, and Wonder Woman, as well as all their friends and allies.
Post-Crisis, Post-Zero Hour
After the Crisis on Infinite Earths, Earth-2 merged with elements of all the remaining worlds (Earth-1, Earth-S, Earth-X, and Earth-4) to form the new post-Crisis Earth (dubbed "Earth-Sigma" by some). All elements of Earth-2 which were duplicates or close parallels of Earth-1 were erased in favor of the Earth-1 versions. Earth-2's Metropolis (as well as Earth-2's Superman and Lois Lane, Earth-Prime's Superboy and Earth-3's Alexander Luthor) survived in a small hypertimeline. Other Earth-2-derived hypertimelines also remain throughout hypertime.
Earth-2 is the world seen in the first DC comics until the late 1940s/1950s, and is thus the first "infinite Earth" that we ever saw.