Earth-One is the home of DC's Silver Age heroes, including the original Justice League of America.
In this world, Superman had a career as Superboy while in his teens, and an array of other superheroes first appeared in the 1950s or later. The Flash was police scientist Barry Allen, Green Lantern was galactic peacekeeper Hal Jordan, Hawkman was Thanagarian Katar Hol, and the Atom was shrinking scientist Ray Palmer.
Earth-Two is the home of DC's Golden Age heroes, including the Justice Society of America...
In this world, Superman, Batman, Wonder Woman, and Aquaman began their careers as adults at the dawn of World War II (concurrent with their first appearances in comics). The Flash was chemistry student Jay Garrick, Green Lantern was magic-powered Alan Scott, Hawkman was archaeologist Carter Hall, and the Atom was pint-sized powerhouse Al Pratt.
Earth-C is the home of Captain Carrot and His Amazing Zoo Crew. This world is populated with anthropomorphic animals...
Earth-S is the home of the former Fawcett Comics heroes, called "Earth-S" for the wizard Shazam, mentor to Captain Marvel. The rest of the Marvel Family included Mary Marvel, Captain Marvel, Jr., and Uncle Marvel.
Earth-Prime is the home of few or no superheroes; this was supposedly "our" world, where the characters of Earth-One, Earth Two, Earth Three And the others, were merely comic book characters. DC editor Julius Schwartz lived here...
1955 - 1963 The Silver Age: Bathound, Bat-Mite, Batwoman
As the golden age of comics drew to a close, Batman and Robin adopted a dog — and put him in a mask, naming him Ace the Bathound. The silver age was more like a silly age with characters like Bat-Mite, a two-foot-tall tubby alien who was a fervent BatFan, dressing up in his version of a Batsuit and interfering with the Dynamic Duo's crime-fighting work in the usual bumbling sidekick manner for nearly four years.
At least the silver age gave us Batwoman, a former circus performer who inherited a fortune and used it to make herself the X Chromosome Crusader. There really wasn't much romance between her and Batman and her yellow-and-black costume could have just as easily been made for Canary Woman. She publicly retired from crimefighting in 1963 and has been mostly forgotten ever since.