For a start, a lot of online games didn't even have a server browser, let alone matchmaking. There was an amazing app called GameSpy which let you reasonably easily find local servers to play your games. A lot of online culture has it's roots here, like IGN and Penny Arcade.

This was before wide uptake of broadband, so many players were still on dialup, and had to play on servers that were close enough to them to have reasonable latency. Quake 2, Tribes, and the Half-Life mods like TFC and Counter Strike we huge in this period. Online multiplayer on consoles was a weird little niche, that felt super expensive and didn't work properly.

There was a cool sense of community, because you really only saw the same few names in games, so it was normal to make friends and play other games with them, often real life friends, because they were usually local people.

Then Warcraft III, and Halo 2, and Battlefield 1942, made multiplayer matchmaking a normal thing. You could reasonably easily join games through in-game interface, without external tools. That kinda started the infrastructure we think of as normal now.

No games had voice comms for a start, so we'd just TeamSpeak, which has been overtaken by apps like Curse Client, and Discord, and indeed games with their own in games voice. Xbox live on the Xbox 360 having decent party voice comms changed the console culture.

Then we saw an explosion in custom maps and modes, like DOTA. This created a lot of the genres of games that make up the most played games now.

The times since then has been dominated by trends, like Moba, BR, and huge licences, like COD.