What is the Commander format that also is called Elder Dragon Highlander (EDH)? Commander is in essence a multiplayer format (also sometimes played 1 vs 1), based on 100 card decks, which may carry 1 copy of each card (With the exception of basic lands and cards that on the card text themselves state you may run any number of them). One of those cards is your commander.
This must be a legendary creature, and every single card that goes in your deck must adhere to that commander’s colour identity. What this means is that each card may only feature mana symbols that are a part of your commander. The commander is placed in a special zone: the Command zone. You can cast your commander at any time you wish, provided it’s legal to do, much like any creature. Whenever it leaves for a visible zone (Exile, Graveyard), you may instead choose to return it to the Command zone. Every time you do so, another 2 is added to it’s casting cost. This is not mandatory, some decks thrive on having their commander in the Command zone. Should the commander be sent to the hand or deck, it does not get the option to instead return to the Command zone, keep this in mind, it’s a very effective way of denying one access to a powerful weapon.
Commander is, the majority of the time, a multiplayer format, and to balance it, a few other rules were added too. For starters, each player starts at 40 life, instead of the common 20. Also, in an effort to make the commanders themselves more important, if you recieve 21 combat damage from one specific commander, this will also eliminate you. This is an efficient way to beat infinite lifegain. It doesn’t matter who controls the commander. If someone steals your commander and beats you for 21 with it, it also works.
There is also a special mulligan rule for commander, but do note that not every playgroup uses it. Instead of the common mulligan where you shuffle your hand into your deck and draw that many cards minus one, here you can lay aside cards from your hand, then refill to 7. If you’re still not statisfied, you can do it again, but this time deduct a card. This is called the partial paris mulligan. Due to it’s potentional for abuse, some playgroups and forumgoers have expressed concern about this rule, but it is used in most playgroups and online.
Finally, Commander features it’s own banlist. You can find it, with other news and things about commander, here.
Why play Commander and What are its costs?
The main reason to play Commander simply boils down to this: It enables you to play with the cards you enjoy. It might not make the strongest deck, or not even something resembling anything competitive – what matters is having fun. And if you have a deck you enjoy, no matter how it’s built, and some people to play with who are happy to help and enjoy the game with you, Commander will guarantee hours upon hours of fun.
Speaking of the costs, you basically can make Commander as expensive as you want. Only the most finetuned lists really need a bevy of original duals, fetches, shocks and all that goodstuff. If you’re just starting out and want to have fun, you can very well make do with basic lands and come into play tapped duals, and then slowly build up a package of the more expensive lands. This also goes for decks in general. Sure, there will be cards that demand a heavy premium, but you can make powerful decks without them. Whatever your budget is, you can get a very good variety of decks, in pretty much every colour. A lot of Commander’s staples have been reprinted a lot, or are from recent, and often opened, sets, or are only a staple in Commander and not in other formats. This means that even for the price of a well tuned standard deck, you could also get 2 or 3 well oiled Commander decks.