Earth-D was the designation for an Earth, and the Universe it inhabited, that existed prior to the Crisis on Infinite Earths.
Earth-D was very similar to Earth-One and had a Silver Age "feel". This universe had its own super-hero analogues to Earth-One, except they are more ethnically diverse. For example an Asian version of the Flash, a black Superman, and a Native American Green Arrow. This universe had its preeminent super-team, the Justice Alliance of America.