I'd say some of the "eras" of game music could almost be broken up by decades. In the 70s, it was mostly 8-bit sound effects, and maybe a few melodies or jingles, which I guess you could call the arcade era. The music got more complex in the 80s, and I think a lot of stuff had definite melodies for different levels, etc, themes, etc. You could say the next "era" of game music was around the time when stuff like Nintendo came out, which is mid to late 80s. There had been consoles before the original Nintendo, for sure, but I wasn't really around for that stuff. If you're going to try to divide things into eras, you may have to be a bit arbitrary, and do it in terms of "generations" of music and game composers.
With stuff like nintendo, then sega genesis, super nintendo, I'd say games had pretty full and memorable soundtracks with higher resolution sounds and fuller, melody-driven compositions than the 70s and early 80s arcade stuff (someone correct me if that's inaccurate). You also had more types of soundtracks because you had more types of games--especially RPG games, which began to use music more like films use music, for things like cues and cinematic sequences, and characters, other than just zone and level area music. That stuff wasn't totally unique to RPGs, because even level-based fighting games had cinematique sequences, but the music in RPG games very quickly became more story-driven. I think this is the era when game music first started to earn a lot more respect and notoriety in a lot of ways.
I'd say now in the current era game music is much higher profile. Game OSTs and composers are kind of a big deal for a lot of people in the gaming community. It's no longer as much of a niche thing, like it was when overclocked remix started. Music now has a much higher production value, and in many games, a more pronounced cinematic influence. In a lot of ways, games are replacing movies as a more interactive cinematic experience, and the music has definitely moved into that territory for a lot of titles.
I'm not an expert by any means, but that's kind of the simplified way I look at it. I'm not well versed in older game music, since I grew up in the 90s, but like I said before, I think the evolution of game music has been pretty fluid, so the "eras" I speak of are kind of arbitrarily drawn up and probably a bit inaccurate. Hopefully some 70s and 80s game music buffs can give you better insight into the early days. Or just better insight period. I think OCR originally started as a spin-off from people remixing Commodore 64 tunes from the 80s.
You should really be looking this stuff up though
Cheers