What makes Eve interesting is that all star systems except for those in the very middle of the map are controllable by players, and actually provide value to those holding them. So that's why you have these gigantic, drawn out conflicts and large battles where player blocs with thousands of players each are fighting for territory.
There are some built-in 'NPC Corporations' that you start on, and default back to if you get kicked out of a Corp, but they're basically just placeholders.
The big alliances are empires that control many aspects of the game. The downside is that if you want to play casually and don’t like all the boot-licking and drama that comes with player corporations you miss out on a lot of the game.