Servers (and channels within those servers) appear on the left rail; the list of server members appears on the right. At the top (or on the bottom on mobile) you can add and message friends, search servers, and see if you've been mentioned in a conversation. A single, free user can join up to 100 servers, while a paid subscriber can join 200.
There are many ways to communicate while gaming online, but many of them are tied to specific hardware. Discord succeeds because it's platform agnostic. You can integrate it with your PlayStation or Xbox account, for example. However, Discord is not locked down to any one system, unlike Xbox Live and PlayStation Network. Not being tethered to a game console also strengthens Discord's argument for non-gaming uses such as talking to family or classmates.
With over a quarter of a billion users, Discord is one of the most popular ways that gamers communicate online. Discord lets friends chat via voice, video, or text, and join servers where larger communities gather.
Gaming is Discord’s focus. At one point, the service gave away free PC games to paid subscribers before realizing it couldn’t beat Steam. However, any online community can take advantage of the features Discord offers. More on its other uses in a bit.