Futbol. Calcio. Futebol. Football. Soccer.
Depending on where you live in the world, you might call Association Football one of these names. Of course, Association Football (which I'll just call "soccer" since I'm in the USA and it's short) has thousands of translations.
And that's what's so amazing about soccer: because it has many translations, everybody around the world knows what the sport is.
For instance, each culture has cuisine, sports, and traditions that don't translate well to other cultures (Think: a foreign holiday that your country doesn't celebrate. Different, right?). But, start kicking a ball around and everyone will be on the same page.
Now, what makes this sport a universal language? Why do so many people get a kick out of playing soccer? (Yes, pun intended.) Mostly, it has to do with the simplicity of the game.
To play soccer, you only need a ball and four markers to make goals. That's it. You don't need an oddly shaped ball aerodynamically designed for throwing or a bat, ball, gloves, and bases. Furthermore, the rules are simple. The object of the game is to put the ball in between your opponent's goal posts; the most complicated rule is the offside rule (I won't get into it here, but it only takes a minute or so to understand). There are no ground-rule doubles, infield-fly rules, encroachment against the defense, or any of that. It's as simple as any game can be.
A lot of people grow up loving some other sport more than soccer, and that's normal. Every person generally loves a sport because they grew up with it some sort of way. Just as there are fans obsessed with the NFL, there are fans obsessed with soccer. But where as most sports have offseasons, soccer does not. Most soccer leagues around the world are year round. Therefore, soccer almost becomes a way of life for most people: it can make a grown man cry when his team loses; it can make a grown man cheer like a giddy school girl when his team scores a goal.
This is even more true in international soccer, especially during the World Cup. In some nations, it seems that everything grinds to a halt to watch they're national team play. In England, companies take extended breaks to watch their boys play; in Mexico, all the TV hosts don the famous green jersey; in almost all countries, cities place large TV screens in the streets so supporters can cheer on their national team.
The unity under the beautiful game is incredible. Last World Cup final in South Africa, people of different races crowded Soccer City Stadium in Johannesburg, even though the final involved only Spain and the Netherlands. People were there for the sheer excitement and love of the game.
Soccer might not be the most popular sport everywhere, but it sure is the most popular overall. But it is due to the fact that the game could almost be considered a culture in itself. And once you're part of that culture, it's hard to let go.