Earth-S was one of the "infinite Earths," home of the Marvel Family and their allies. During the Crisis on Infinite Earths, it was temporarily stored in the netherverse created by the Monitor's death, then covered in ice and conquered by an army of super-villains.
Post-Crisis, Post-Zero Hour
After the Crisis on Infinite Earths, Earth-S merged with elements of all the remaining worlds (Earth-1, Earth-2, Earth-4, and Earth-X) to form the new post-Crisis Earth (dubbed "Earth-Sigma" by some). Other Earth-S-derived hypertimelines may also remain throughout hypertime.)