It used to be called Elder Dragon Highlander. It’s now called Commander. We’re probably going to keep on calling it EDH for a long time to come. You can read the detailed rules and philosophy at the Elder Dragon Highlander/Commander Rules website, but what’s a little recap between friends? Here’s the quick and dirty version, just for you.

1. Your EDH deck must consist of exactly 100 cards.
2. It’s a singleton format, so no two cards can have the same name, with the exception of basic lands. (THERE CAN BE ONLY ONE. *cue Queen*)
3. You have to pick a legendary creature as your General (now also known as the  Commander).
4. Your Commander has a color identity. This includes the mana symbols in its mana cost and in its rules text. This color identity dictates the colors of the mana symbols that can appear in all of the cards in your deck, either in their mana costs or their rules text . Note, however, that color words, such as “black creatures” or “blue spells,” aren’t part of the color identity. Neither are references to specific land types, such as Swamp or Island. Therefore, [card]Bringer of the Red Dawn[/card] would be illegal in an [card]Ashling the Pilgrim[/card] deck, but [card]Ember Gale[/card] would be just fine, and so would [card]Arid Mesa[/card]. If you want to use a split card, both sides of the split card must conform with the color identity of the Commander.
5. Your deck can’t generate mana outside of your Commander’s color identity. If some effect you control would generate an illegal color, it produces colorless instead.
6. The Commander starts out in a special outside-of-the-game zone called the Command zone. This is not the same as the Exile zone. You may cast your Commander from that zone any time it would be legal for you to do so.
7. If your Commander would be exiled or put into a graveyard from anywhere, you can opt to plop its ass back in the Command Zone.  It costs 2 colorless more to play for each time you’ve cast it from the Command Zone.
8. Players start with 40 life.
9. Any player dealt 21 or more points of combat damage from a Commander loses the game. Note that it’s specifically combat damage—sorry to disappoint, [card]Niv-Mizzet, the Firemind[/card] combo players everywhere. [card]Uril, the Miststalker[/card] players, however, rejoice. And yes, you can be beaten to death with your own Commander. It’s happened to most of us at some point or another. However, Commander damage is specific to a particular player/Commander pairing, which means that if somebody has hit you with 20 points of Commander damage, and somebody else steals that Commander and continues to whale on you with it, they still need to deal you another 21 to achieve lethal Commander damage. Also note that life gain does nothing to take away from Commander damage; once it’s done, it’s with you forever. Also note that a Commander remains a Commander even after it’s been transformed into something else, like, say, [card]Lignify[/card] or [card]Turn to Frog[/card], or if it’s been turned face down by [card]Ixidron[/card]. And as a corollary, if somebody else plays a non-Commander creature that happens to be the same as your Commander, or if somebody copies your Commander, those cards aren’t Commanders, can’t copy the Commander characteristic, and can’t deal Commander damage.
10. Some cards break the format and result in unfun experiences for the community. The EDH Committee maintains a short list of recommended banned cards (click on the “Social” link at the Elder Dragon Highlander/Commander Rules site). Most playgroups follow them, sometimes with modifications based on their meta.

And unfun plays leads us to the Golden Rule of EDH: Don’t Be an Ass. We’ll be writing  a lot more about  the social aspects of the game and what it means to be (or not to be) an ass, but what it boils down to is this: EDH is a casual format, and its explicit goal is to foster creative, interactive gameplay. It’s meant to allow you to play the big, silly spells that would never see play in the sanctioned constructed formats, in truly epic game settings. The kinds of games where you get to swing with a [card]Chameleon Colossus[/card] so huge, you have to resort to scientific notation to keep track of its power and toughness, or in which you give your 120/120 [card]Lord of Extinction[/card] Horsemanship. It’s the format in which you get to indulge your Timmy urges. It’s the format in which [card]Tarmogoyf[/card] inspires snorts of derision, but a [card]Yavimaya’s Embrace[/card] may win you the game. It is, in short, one of the most accessible and fun formats of Magic out there. We love talking about it, and we love playing it even more.