And now? For a growing wave of young people, the obsession is Twitch, the live-streaming platform that feels like the internet’s chaotic, endlessly entertaining living room. But what is Twitch?

Live streaming first gained traction on YouTube, but the platform that truly transformed the streaming landscape is Twitch.

The gunman in a mass shooting that killed 10 people at a supermarket in Buffalo, New York, on Saturday live-streamed the attack on Twitch, the platform confirmed. "We are devastated to hear about ...