When I was first introduced to the game of Magic: The Gathering, I was only aware of Standard and Modern formats, both of which use a 60+ card deck, with a maximum limit of 4 copies of each card other than basic lands. The former of those two formats is further restricted by using only cards from the most recent few releases and requires constant updates every few months to remain current. The cost necessary to play these formats competitively was prohibitive, but I was reluctant to pursue Commander. I finally took the plunge a couple years ago and I have never looked back. Anyone who plays should consider building a deck and giving it a try.
Also known as "Elder Dragon Highlander," or EDH, Commander began as a homebrew format for casual players. The "Highlander" part of the name comes from the eponymous films and TV series and its motto of, "There can be only one." This is because a Commander deck is a "singleton" format, meaning you can only have a single copy of any card that isn't a basic land. One of the cards must be a legendary creature that serves a special function as your general in the game. In the original version, these were Elder Dragons, thus explaining the rest of the name.
Wizards of the Coast, the publisher of Magic: The Gathering, later adopted it under the name "Commander," and has formally supported it with annual releases for several years. However, any player can build a deck with cards from even a fairly modest collection, or modify a prebuilt deck to better suit their preferences.
The basic rules are fairly simple for anyone already familiar with standard Magic. Other than the singleton deck requirements, the major changes are that you have 40 life instead of the usual 20, your deck can only contain cards that share a color identity with your commander (and vice-versa), your general is kept in the "command zone" and can be returned there whenever he would otherwise leave the battlefield to be re-cast for a cumulative cost penalty, and any player dealt 21 or more damage by any individual commander creature loses the game regardless of his life total. The banned cards list and further details can be found at the official community site.
Legendary Creature cards can range in cost from less than $1.00 to over $100. Competitive decks can be built with cards from quite low on that price scale though. Part of the fun in building a Commander deck is finding the best way to exploit your commander's abilities to enhance the deck, or add cards to enhance your commander to get a win through commander damage. As such, cards that are considered fairly worthless in Modern and Standard can be quite useful (or a fun challenge) for building a Commander deck. Since only a single copy of any given card is needed, there is no need to spend the effort to get extra copies of a really good card, and the one copy you may have found in a booster pack is all you need. A tribal deck is especially fun, since you can use 30 different unique humans, goblins, elves, soldiers, angels, rogues, or other race/class archetype you find interesting and it gives you an excuse to collect more of them.
In future posts, I plan to link to deck lists I have built and explain how they work along with their strengths and weaknesses along with some strategy discussion. I also plan to cover some interesting variants on this format that have been developed by the community. Commander began as a fan-built format and continues to evolve because of the fans. That's why it's the best.