The answer to your question is very complicated. From the dawn of continuity until presumably the end of superheroes' first popularity wave, there were certain stories which were later revealed to exist on Earth-2. The original Earth-1 was the Earth stories took place on probably starting about the time Barry Allen was introduced. When Allen met Jay Garrick, the Golden Age Flash, who on Earth-1 was a mere comic book character, it introduced the multiverse. This is when Earths 1 and 2 got their designations. The multiverse spread to include Earth-3 of the Crime Syndicate, Earth-4 of the Charlton Comics characters, Earth-S of the Fawcett Comics characters, Earth-X of the Quality Comics characters, Lady Quark's Earth-6, and so on and so forth. One of these was the original Earth-Prime. Many say Earth-Prime is our Earth, that we, the readers exist on. I disagree. While it is true that a company called DC Comics publishes the adventures of their characters on Earth-Prime, and for the most part, none of the characters really exist on Earth-Prime, there is the existence of Superboy-Prime (and Ultraa). While Earth-Prime shares many similarities with the real world, I believe that writers of the DCU do not have the power to write something into the real world, so unless there really is a Superboy in our real Kansas somewhere, I maintain that Earth-Prime is not quite our world.
Anyway 1985's original Crisis merged all Earths (primariliy Earths 1, 2, 4, S, and X) into one post-crisis Earth. Due to this, the need for designating Earths by number was lost. However, this sole Earth was post-crisis Earth-1. (The word post-crisis is important.) 2006's Infinite Crisis however, reformed the multiverse, however, instead of housing a literally infinite number of universes, it was strictly limited to 52 universes. The other consequence of this was that the mainstream DCU's name was changed to New Earth. In the Crisis on Two Earths movie an Earth Owlman calles Earth-Prime is the center of the multiverse, the item whose destruction causes the destruction of the entire multiverse. This Earth-Prime is not related to any Earth-Prime from the comics. The movie people probably just liked the term. However, New Earth is the center of the multiverse in the comics. Its destruction unravels the entire multiverse. (See Sinestro Corps War.)
In the year-long weekly series Trinity, a new Earth is created and designated Earth-1. This is presumably the Earth the Earth One OGN's take place in. It's relationship to the rest of the multiverse is unclear due to its unique origin, and the strict limit of universe allowed in the multiverse of the time. (Though several universes were wiped out, so I guess it could have replaced on of those.) My guess is that this new Earth One was given the name because the mainstream DCU had settled on the name New Earth for the time being.
Anyway, in 2011 at the end of Flashpoint, the New 52 happened. The New 52 Earth is now the main DCU. (in that it is not an alternate Earth from pre-Flashpoint because Pandora and the Flash altered reality to create it.) It is fairly apparent that this no longer designated New Earth though, and that the limit of 52 universes in the multiverse has been thrown out the window. The problem then becomes, if Earth One is the Earth the OGN's take place in, what to call the mainstream New 52 DCU? I've hear both Prime Earth and Earth-0 used to refer to it, but have not heard any official confirmation from the publisher. My hope is that they will stick with Earth-0 because it's the only name that can't be confused with other things. So Earth One can refer to anything from 1959-2006 basically (with pre- and post- 1985 Crisis subparts) or the OGN's. Earth Prime usually refers to our Earth or an Earth very similar to ours that is home to Superboy-Prime. However either one of these terms may be used to reference the mainsream New 52 DCU, depending upon who's using them, until the publisher designates the universe a proper name. I myself used the name Earth-1 referring to this in another thread on this board because Earth-3 was in question and I presumed using the term Earth-1 would be fairly understandable. I myself do not like the term Prime Earth due to its similarity to the name of Superboy-Prime's Earth. I hope you find this post helpful. I do however realize that this is a lot to digest. So take your time.
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