League of Legends The most-played PC game in the world isn't the Sims or SimCity or even World of Warcraft. It's League of Legends, and it's a lot of fun.

Part of this game's success is that players can compete in tournaments against other players from around the world. At the 2013 championships, the top two teams gathered in front of thousands at the Staples Center in Los Angeles. A team from Korea, called SK Telecom T1, took home the $1 million prize.

In 2013, it had 70 million registered users, up from 15 million in 2010, according to IGN. And of those 70 million registered users, it had 32.5 million active users. Together, they played a massive 1.3 billion hours each month.

The company's success comes in part due to its business model: It doesn't charge people to play. Once you're in the game, however, you can choose to pay for certain characters or for cosmetic items, such as new costumes for the characters.

These payments have helped Riot Games, the company behind League of Legends, bring in more than $624 million in revenue last year, writes Ian Sherr of the Wall Street Journal.

But just what is it that they're playing? Let's take a look at the basics and find out. (If you really want to dive deep in the gameplay, check out this beginner's guide.)