Earth-1: The Silver Age. Justice League, Superman II, Flash II (Barry Allen). Largely identical in history to the real world. Almost all post-Golden Age comics through the Crisis took place here, beginning between 1945 (first appearance of Superboy) and 1955 (first appearance of the Martian Manhunter). First defined in Flash v1#123 (1961), first named in JLA v1#21 (1963).
Earth-2: The Golden Age. Justice Society, Superman I, Flash I (Jay Garrick). Largely identical in history to the real world up through the mid-70s, at which point minor differences creep in (such as South Africa becoming free decades early). Only a few post-Golden Age comics were ever set there, notably Infinity Inc., the second run of All-Star Comics, and All-Star Squadron. "Defined" and "named" info same as for E1.
"Reversed" Earth. Crime Syndicate, Ultraman, Johnny Quick II. First
suggested in JLA v1#22, first appeared in JLA v1#29, destroyed in Crisis
Earth-4: Charlton Comics. Captain Atom, Blue Beetle. First DC appearance and named in Crisis #1. Earth-S: Fawcett Comics. Captain Marvel (Billy Batson). First appearance in Whiz Comics #2 (1940), first DC appearance in Shazam! #1 (1973), first named in JLA v1#135 (1976).
Proposed name for the world seen only on pages 2 and 3 of Crisis #1. No
apparent superheroes. (Note: The real reason there is no
explicitly-named Earth-5 is that "5" and "S" look too much alike,
particularly when hand-lettered, so that number was skipped.)
"A cosmic anomaly." Lady Quark, Lord Volt, Princess Fern. Only
appearance in Crisis #4. Earth-K: The name given to the future timeline
of Earth-1 which Kamandi inhabited, to distinguish it from the "real"
timeline which led to the Legion of Super-Heroes. First appearance in
Kamandi #1, last appearance in Crisis #4. (Technically an alternate
timeline, not an alternate universe.)
Earth-X: Quality Comics. Freedom Fighters, Uncle Sam, the Ray. Noteworthy in that World War II continues into the 1970s. First DC appearance and named in JLA v1#107 (1973).
Qward: The antimatter universe which contains the planet Qward is usually called "Qward" itself. It is the home of the Anti-Monitor. First appearance in Green Lantern #2 (1960). Earth-Prime: In theory, the "real world". In actuality, merely a variant Earth with very few superbeings, in which most DC Comics characters are just comic book characters. Devastated by nuclear war in the late 80s. First appearance in Flash v1#179 (1968), named in JLA v1#123 (1975). Destroyed circa Crisis #10. (Does not technically appear in Crisis, but is referred to.)
Earth-Omega: Proposed name for Pariah's home universe, the first one destroyed by the anti-matter wave. Only appearance in flashback in Crisis #7.
Earth-Sigma: Proposed name for the post-Crisis, pre-Zero Hour universe. "Sigma" for "the sum of what came before". Merger of Earths 1, 2, 4, S, and X, with characters from at least 2 others present. First appeared in Crisis #11, destroyed in Zero Hour #1. Earth-D: The Earth that appears in - and is destroyed in - Legends of the DC Universe: Crisis on Infinite Earths (1998). It features an ethnically diverse range of heroes, and is rumored to be what Wolfman thought the DC Universe should have been like after the Crisis.