The Worlds of the 52 multiverse were created by DC Comics as part of thier Infinite Crisis/Final Crisis series of comic book events.
While many of the worlds resemble existing worlds, it has been made clear by writers and edits that they are not direct copies of them
New Earth
This is the main universe, and contains most of the characters and stories since Infinite Crisis.
It is a continuation of the "main universe" which has been published in various stories by DC Comics since in 1939.
Earth-1
In the comic introducing the 52, the character Rip Hunter mentioned that New Earth and Earth-1 are separate alternate universes. Nothing else is know about this Earth. There are some references to New Earth as Earth 1, but they are generally considered mistakes by writers. Due to Earth 2 resembling Earth Two there is some speculation that it resembles Earth-1.
Earth-2
Resembles Earth-Two, and a newspaper article says that this world's Superman and Powergirl are missing. This reflects Earth Two Superman's death in Infinite Crisis, and the revelation during Infinite Crisis that Powergirl came from Earth Two.
Earth-3
Resembles the pre-Crisis Earth-Three, the Antimatter Earth and JLA:Earth-2. This Earth is home to the Crime Society of America which has been described as a "world full of evil doppelgangers of Earth-2."
Earth-4
Resembles the pre-Crisis Earth-Four. Described as a film noir world populated by alternate versions of the Charlton Comics characters that uses story elements from the Watchmen limited series
Appearances
* 52 Week 52 (single panel)
Characters
* Characters purchased from Charlton by DC.
Earth-5
Resembles the pre-Crisis Earth-S. Earth-5 was first used to name an analogue for Earth-S during Infinite Crisis