Fans flood arenas wearing their team jerseys. They rush to events in search of players’ autographs. But these “athletes” they’re flocking to see, who spend hours in scrimmages honing their skills, are not football or basketball players — they’re gamers.

Competitive video game playing, more commonly known as esports, drew 258 million unique viewers globally last year, according to research firm SuperData. For perspective, the National Football League said 204 million unique viewers tuned into the 2016 NFL regular season in the U.S., based on Nielsen data.

The esports business surged in recent years with the arrival of streaming services like Twitch capable of broadcasting players’ video game exploits to thousands of viewers.

Just like “real” sports, esports makes money off of investments, branding, advertising and media deals, raking in $1.5 billion in revenue last year, said SuperData. The firm expects the esports industry to hit 299 million viewers this year and top $2 billion in revenue by 2021.

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“It’s growing at a pretty steady pace, well in to the double digits year over year, which is very healthy,” said SuperData CEO Joost van Druenen.

Several esports leagues have surfaced, luring investors from traditional sports leagues including former NBA star Rick Fox, owner of esports organization Echo Fox, and New England Patriots owner Robert Kraft. Even the International Olympic Committee is reportedly considering adding esports to future Olympic Games.

The latest to launch: the Overwatch League, which kicked off its inaugural season Jan. 10 with 12 teams from cities in the U.S., China and South Korea that will compete against each other in the competitive video game Overwatch. The league will feature regular season matches from a custom-built arena in Los Angeles, leading up to playoffs and a championship event.

“Esports for me is just as, if not more, entertaining as regular sports,” said Jake Duchesne, a 22-year-old fan who says his love of esports was sparked in the same way he started following the NFL, NBA and NHL — he wanted to watch “the best of the best” compete in the sports he enjoyed.

“I have a favorite team, players that I enjoy watching succeed and players that I dislike,” said Duchesne, who is a student at Arizona State University.

Duchesne also regularly attends events, including the Call of Duty Championship and the League of Legends League Championship series.

How did esports become such a thing? Here’s a primer on this growing slice of the video game industry:

Esports basically turns video game playing into a sport. In some scenarios, it’s on an individual level, but mostly it’s through teams.

The games themselves are like different sports types, in some ways. For example, in esports, Call of Duty and League of Legends are equivalent to, say, football and baseball. Esports is the broader concept, while the games are the individual types of sports.

Video games with a strong competitive element fit best as an esport. For example, the Overwatch League is based on the online first-person shooter Overwatch, which features a series of competitive match types.

Fans of the games and teams will watch hours of competition, either through streaming services like Twitch or attending live events.

For esports athletes, as for any competitor, it’s about being the best of the best. Of course, the financial incentives are getting bigger, too. The International, a tournament featuring the game Dota 2, boasts one of the biggest prize pools at $24 million, with the winning team splitting more than $10 million. Last year’s League of Legends World Championship had a nearly $5 million pool. This doesn’t factor in endorsements or potential sponsorships, which are expected to grow over the next several years.

Because of the money involved, more game publishers have entered the market, such as Activision, Electronic Arts with its sports titles Madden NFL and FIFA, and Turner Broadcasting, which launched ELeague nearly two years ago along with WME | IMG.

League of Legends (LoL). Publisher Riot Games is the most popular title in esports, a multiplayer online battle arena game where players work together to take down their opponents’ nexus housed in the center of their base. There are multiple paths to both bases, which means teams will try to strategize the best approach to their opponents’ base, as well as techniques to protect their own. According to research firm SuperData, it boasts more than 100 million monthly active users. Riot runs events year round, and announced last summer it would adopt a franchise model similar to Overwatch League for its North American championship series.

Dota 2. It’s similar in style of play to LoL, but run by publisher Valve Software, the same company behind the Steam games marketplace for PC players. Although its player base is smaller (more than 12 million, according to SuperData), it hands out more lucrative cash rewards. It runs The International, with a prize pool of more than $24 million during last year’s tournament. There are also several third-party tournaments hosted worldwide.

Counter-Strike: Global Offensive. In terms of longevity, the Counter-Strike series is among the top games in esports. Some of the game’s match types are objective-based, where one team must complete a task and their opponent must stop them. Several third-party organizations run their own tournaments, including Major League Gaming, ELeague and ESL.

Overwatch. Through the Overwatch League, game developer Blizzard and publisher Activision will attempt to run it like a traditional sports league, with city-based teams and ownership, regular seasons and playoff periods. It’s a first-person shooter, but with a strong focus on team play, featuring tasks such as escorting a payload to a destination or protecting/attacking key points on a map.

Arguably the biggest is respectability. Some people don’t see athletes. They see people playing a video game. For example, on January 9, former sportscaster Keith Olbermann criticized the sports website The Players’ Tribune for “publishing pieces by snotty rando kids playing children’s games,” after the site ran a feature on esports star Yiliang “Doublelift” Peng. Olbermann took some heat on Twitter for that one.

Keith Olbermann tweet

Meanwhile, debate still lingers over whether professional gamers should be considered athletes.

Duchesne said pro gamers have an advantage over traditional athletes because they seem to interact more regularly with fans through streaming or social media.

“There’s a level of closeness with esports players that I don’t believe is as possible with professional traditional athletes,” he said.

Another challenge: creating a broadcast experience so viewers can easily follow along, similar to when poker added glass to the tables so viewers could see what cards players held. “That’s makes it interesting and exciting because we have kind of an inside view and we know what’s going on,” said Wedbush analyst Michael Pachter in an interview in 2016.

Beyond the financial stakes, it also captures a much younger audience coveted by advertisers. A 2016 market report from research firm Newzoo found more than half of the esports audience is between the ages of 21 and 35, while 27% is between 10- and 20-years-old.

Plus, esports are fun for the same reason traditional sports are: it’s really about the spirit of competition. Just like we love watching Tom Brady, LeBron James, or Giancarlo Stanton playing at the highest level, fans of video games and esports enjoy watching the best players in games like League of Legends or Dota compete.

As esports grows with its fans, it could inch closer to reaching more mainstream acceptance.

“There’s a generation of sports fans growing with esports as their primary sport of choice,” said Whalen Rozelle, director of esports at Riot Games, the creators of League of Legends, during an interview with USA TODAY in 2016. “They’re not dipping in and watching basketball, hockey or football. This is a generation that really focused in on this as their sport.”