No idea what Discord actually is. Heard people use it but have never seen anyone use it personally.
Its a chat app put simply, but its basically the best of every chat app imaginable.
You have persistent accounts that can connect to multiple servers through a multitude of sources (web, mobile, desktop). Servers can have multiple voice chat rooms, multiple text chat rooms, api integration for bots, deep permissions mapping. User tagging in messages, room pinging, and a ton more.
I used it for text chats LONG before I ever migrated for voice.
When I was still playing fighting games I had servers for games, and individual servers for discussing characters I played, alongside my local friends group room. The UI in all versions of the app is fantastic and makes it a breeze to bounce between multiple conversations.
As a text chat app alone, it really puts to shame a lot of the long standing chat apps like Line.