Earth-X was conceived as a way to recover Quality Comics characters that DC bought years ago. Using the concept of parallel universes, the JSA and the JLA would help Uncle Sam to return to an earth where the Nazi Germany won World War 2 and then conquered the USA. Most of the adventures of the Freedom Fighters would be set in this earth.
Earth-X was created by Roy Thomas and Jerry Ordway
Bronze Age / Earth-X
After Crisis on Infinite Earths, Earth-X was clustered in the main Earth.
Post-52 Weeks / Earth-10
Post-52 Weeks, Earth-X was renamed as Earth-10. The main difference between the Pre-Crisis and the Post-Infinite Crisis/52 Weeks, Earth-10 is the existence of a Nazi version of the Justice League called JLAxis, who oppose to the efforts of the Freedom Fighters from topple the Nazi powers.
Post-Final Crisis / Earth-10
Earth-10, is a world where the Nazis have won WW2 and has control over the rest of the world. The Nazis' has a super-power team called the New Reichsmen, who opposes the terrorist group the Freedom Fighters and their leader Uncle Sam. During Final Crisis, Overgirl cross the Bleed barrier between the Multiverse. She landed on New Earth, where she die there.