"Earth-Zed" is essentially an Earth that takes place between 1994 (the year of "Zero-Hour") to 2005 (the start of "Infinite Crisis"). There was some disagreement as to whether or not Zero-Hour should factor into anything, based upon the Metaverse Method. However, I contend that Earth-Zed (or "Z", for Zero) changed things in the DCU in a fundamental way:
1) introduced the "ten-year" time frame for an active Superman, and thus affecting everything else.
2) introduced the concept of "Hypertime" to the DCU
3) changed origins for some characters (such as changing "SHAZAM!: A New Beginning" into "Power of SHAZAM!").
4) introduced a revised timeline for the DCU.
Of course, whether or not having an "Earth-Zed" is up to debate. But, I wanted to put it out there for discussion.