A Parallel Earth exists in a parallel universe. Travel between earths is possible by "slide"-ing through a wormhole referred to as an "Einstein-Rosen-Podolsky bridge."
Many of the worlds the Sliders encounter have alternate histories to that of Earth Prime. Allowing the group to explore what would have happened, for example, if America was conquered by the Soviet Union, if Britain had won the American War of Independence, if penicillin had not been invented, or if men were subservient to women.
On many parallel Earths, there will be alternate versions of the same person. The Sliders frequently encounter alternate versions of themselves or people they knew on Earth Prime.