m not exactly a comic book history buff, but I can give you a basic overview.
The Golden Age is what they call the years between 1938 (when Superman first appeared) and the late 1940s or early 50s. Stories in that era carry a lot of morally questionable content (Superman smashes up slums to get the land lord to build better homes; Batman shoots people dead with guns) and wartime propaganda. (This is where we get classics like "Superman Says You Can Slap a Jap!") Other than Superman, most heroes were powered by magic.
The Silver Age is from the mid 1950s to the 1970s; it's characterized by science fiction stories and bright optimism. Even Batman was out doing crazy things all the time. It relaunched the Green Lantern and Flash into the form we know them as today, and also saw the birth of Spiderman, Hulk, Fantastic Four, Iron Man, and the frozen-in-ice rebirth of Captain America. Marvel first started making superheroes unique and reasonably realistic people in the middle of the silver age, an approach that DC quickly emulated. Unfortunately it was also the era of the Comics Code Authority, which had a ridiculous amount of regulations to comic content. Jack Thompson would salivate at the prospect of putting something similar on video game content.
The Bronze Age began in the 1970s and is sort of a neo-golden age with better art. A lot of the innovations of the Silver Age remain, but the stories got a lot more serious, especially Batman. He stopped going in to space and Robin showed up a lot less. Batman: The Animated Series is a great example of Bronze-Age storytelling. The art was also more realistic then, because guys like Jim Aparo and George Perez were at the height of their careers.
The "Modern Age" began in 1985 when DC rebooted its continuity with 'Crisis On Infinte Earths', which was basically a cosmic retcon that let them start over. In the new world, Superman no longer had powers as a kid, Lex Luthor was a business man, etc. Marvel and DC also got a lot darker, and the Comics Code Authority became basically irrelevant. Also, 1986, DC published what are largely considered two of the masterworks of the medium, Dark Knight Returns and The Watchmen. Modern Age stories also have much cleaner and more realistic coloring thanks to computer image manipulation.
And that's everything, but that's just the gist of it.
As for Earth-1 and Earth-2, Everything before 1955 is set on Earth 2, everything between 1955 and 1985 is set on Earth-1.