Think nostr relay chat but centralized. Discord basically gave everyone free chatrooms and voice chat rooms for their own communities. Since it was better than teamspeak and also didn't require you to get your own paid server it became very popular. We have a discord server for our AI software for example where our users can hang out with each other, discuss local AI models, discuss the software and interrate on development that sort of thing.

The culture in a server depends immensely on what who made it / moderates it. So theres no specific thing there as theres not really a central big server everyone has in common.