A children's card game based off of a children's TV Show and played predominately by adults
If the backstory is to be believed Yu-Gi-Oh! is based upon a game played by Ancient Egyptians, thousands of years ago.
If, however, you're grounded in reality and instead choose to not believe that load of horse faeces, then Yu-Gi-Oh! is based upon the fictional game of Duel Monsters that appears in Kazuki Takahashi's manga and anime series by the same name. It's made by KONAMI.
The card game is outstandingly popular (more on this later) due to it being the only card game in the world to recieve half an hour of dedicated product placement worldwide. This is cleverly disguised as the Yu-Gi-Oh! television show which is currently in the midst of its third series (not season, series).
This means that there's more episodes of Yu-Gi-Oh! out there then there is porn on your hard drive (because I seriously doubt you have more than 120GB of porn on your hard drive but hey, go ahead, prove me wrong). Yes, that's right, Dark Magician Girl's 2D Anime tits are more popular than all the real ones you've amassed over the course of your porn watching life.
Look at them, trumping you.
The game is based around summoning monsters to attack your opponent's life points in an effort to reduce their score to 0. The fact that this mechanic was called "summoning" was enough to incite some christians and get some religious kids worried about going to hell.
Soccer mum's standard paranoia aside, you summon monsters of a select few types: Normal, Effect, Fusion, Ritual or the new Synchro.
Normal monsters got nicknamed Vanilla because the border colour on the card is the colour of good quality vanilla ice-cream
Like this, but with dragons.
The other types currently see far more play and are ultimately more useful. However they lack an ice-creamy nickname so it's kind of 50-50 as to what more awesome.
Spells and Traps round out the card types. They don't really do much on their own and as a general rule a hand of all monsters will beat a hand of all spells and traps. Basically they just have effects that help your monsters be awesome. It's kinda like how Batarangs on their own are just pointy bat shaped metal. However give them to Batman and they make him even more awesome.
Despite being marketed at children, the competitive scene is dominated by people who live with their parents not because it makes primary school easier, but because it makes their work life easier.
These guys turn up regularly for local tournament. Basically, this is like your local soccer club in terms of bragging rights. Yes, you guys may be the undefeated champions, but you're still a local freaking soccer club.
Ironically, players tend to bitch and moan just about as much as competitive soccer players. Except instead of taking dives, Yu-Gi-Oh! players accuse the opponent of stacking their deck and when we throw hissy fits we have the aforementioned deck of cards to throw all over the place first.
I Struggle to tell what game this guy is playing.
The next step up from Locals are Regionals. These are qualifier events for the national competition. These are intense competition... inside the USA and Japan. Everywhere else the small turnouts result in the top quarter of the competition qualifying entirely on the basis that they turned up and beat all the people that haven't worked out that "Blue Eyes White Dragon" actually isn't any good at all yet (read: 9 year olds).
After that we have Nationals. You get here from regionals. This is supposed to be the pinnacle of your country's playing ability. In the weeks that follow this event, all the player in your country will proceed to complain about the guy that won and how someone else was "totally screwed out of it" solely on the basis that they weren't the aforemention champion.
The lucky a**hole
Then there's Worlds. An event that for some reason no-one really cares about. Seriously, any competitive player can tell you the names of the guy that won their nationals each and every year they competed but no-one can get more than one World Champion out. Also, for whatever messed up reason, Italy and New Zealand do ridiculously well but everywhere else stands around looking sheepish.
Get it? 'Cause New Zealanders screw sheep.
Yu-Gi-Oh! in June of last year was declared by the guiness book of world records as the highest selling card game in the world with over 22 billion cards sold world wide. That's over three times the world's population and I guarantee you that the number of people playing the game is nowhere near that.
The deadly combination of children's card game and anime boobs has sent the popularity skyrocketing.
This popularity has caused something of a crazy turn of events. The cards can become ridiculously expensive. This reached a peak (I don't want to say it's peak, because that implies it won't happen again) during what was known as Dark Armed Dragon Format when competitive players would pay upwards of $5000 dollars to make their decks.
Now if that last fact doesn't make you sick, here's one that will:
This thing makes me want to puke
In December 2004, the second ever copy of the above pictured "Cyber Stein" printed in English went up onto eBay. It sold for a cool US$23,000. The actual listing for this item has been taken down since then but here's some people going crazy about it instead.
Allow me to help that sink in. Someone, somewhere paid $23,000 for a piece of cardboard with some coloured foil in it. That means someone consciously chose this over a car. The latter being FAR more likely to get the purchaser laid.
There is allegedly a "Japanese Blue Eyes Ultimate Dragon With Armor" floating around somewhere that will set you back a cool $500,000. Y'know, the cost of a house. No-one on the internet has any proof it exists though so I'm going to stick with the Stein as the most expensive card.
And this is supposed to be a children's card game.