Discord competes with Teamspeak, Mumble, Ventrilo, and Skype in the category of gaming voice chat, but it also offers an interface set up like Slack, which has gained popularity for topic-based persistent chat. It hits a sweet spot of good-enough voice & text chat that's cost-free, needs no configuration, is fully hosted with captive servers, uses decent codecs, supports inbound hyperlinks, has a slick UI, has official same-name clients on mobile, has a web client, and the like.

Discord's rise helped by Microsoft's seeming neglect and confusing platform direction of Skype in recent years, even if Discord still doesn't have video chat. For messaging among people who know each other by name, Facebook's offerings are dominant, but for pseudonymous chat Skype remained strong until Microsoft vacillated between making Skype Windows' native messenger or a strong multiplatform freemium network that isn't plagued by an aging UI and the invasion of advertising. Discord won many users by organic word-of-mouth referrals from people who used to use Skype.

As for gaming voice chat, Discord is not as sophisticated as other offerings (edit: maybe that's the wrong wording; let's go instead with "not as configurable"), but offers a smoother out-of-the-box experience.