Reports broke on Sunday evening that YouTube, the uncontested leader in online video, was in talks to buy Twitch, a relatively niche streaming service, in a deal totaling more than $1 billion. The Wall Street Journal quickly amended Variety’s original report and claimed that talks between the two companies were only in the early stages, but the possible purchase would be YouTube’s most important yet. So, what is Twitch?

Twitch is a service for livestreaming video games (or more accurately, video footage of those games). Users host streams and usually offer live commentary as they play, reacting to the game in real time. This genre of online video—Let’s Play—is incredibly, and increasingly, popular.

As of February, Twitch boasted 45 million monthly uniques. The newest consoles from both Microsoft and Sony even have livestreaming capabilities baked in, whereas older console feeds need to be run through a computer. One of the most popular, if not the most popular, YouTube channels is hosted by a 24-year-old Swedish gamer who goes by the nickname of PewDiePie.

According to Polygon, “in an average month, Twitch broadcasts more than 6 million videos and fans watch 12 billion minutes of footage.” In terms of percentage of peak U.S. internet traffic, Twitch ranks fourth, ahead of Facebook, Amazon, and Hulu.

What makes Twitch’s massive success interesting is the strictness in terms of what can be broadcast on the service: Twitch is only for video games. Last fall, a PlayStation 4 game that made use of the console’s camera for augmented reality was banned from the service because people were using it for purposes other than streaming game footage. That doesn’t mean people haven’t gotten creative with the service, however. In February, someone set up a stream of a Pokemon emulator that took commands from the stream’s accompanying chat window.

A YouTube purchase of Twitch would mean a consolidation of pretty much the entire Let’s Play ecosystem, but it would come with a few complications. Microsoft sewed up exclusive Twitch integration for the Xbox One, so an ownership by YouTube (and therefore, Google) could put that relationship on shakier ground.

On top of Twitch-specific dealings is the looming dubious legality of streaming games at all, the copyrights of which are generally owned by their developers and publishers. Last July, Nintendo nearly prevented Super Smash Bros. from being streamed in a tournament but reversed its stance after outcry. At the beginning of this year, YouTube encountered its own backlash when it pulled thousands of videos of game footage from its service under claims of copyright infringement.