One key to the continued health of Magic is diversity. It is vitally important to ensure that there are multiple competitive decks for the tournament player to choose from. Why? If there were only a single viable deck to play, tournaments would quickly stagnate as players were forced to either play that deck or a deck built specifically to beat it. In addition, different players enjoy playing different types of decks. If there are plenty of viable options to play, there will be more players at more tournaments.
To help maintain the diversity and health of the Magic tournament environment, a system of banned and restricted lists has been developed. These lists are made up of cards that are either not allowed at all, or allowed only in a very limited manner.
It's important to note a couple of things: First, these lists apply only to Constructed formats and not Limited formats. Second, the banned and restricted lists are format-specific, so a card that is banned in Modern may still be legal to play in Standard.
If a card appears on the banned list for your chosen format, then you may not include that card in your deck or sideboard. Doing so makes your deck illegal to play in any sanctioned tournaments for that format.
Cards are usually banned from play if they enable a deck or play style that heavily skews the play environment. What does that mean? If the card were legal, a competitive player either must be playing it, or must be specifically targeting it with his or her own strategies.
Some cards are banned because they have proven to simply be too powerful in their respective format. While hundreds of hours are spent rigorously playtesting sets before their release, the complexity of Magic makes it nearly impossible to accurately predict all the ways the new cards interact with older ones.
Cards whose art, text, name, or combination thereof that are racially or culturally offensive are banned in all formats. This list is a work in progress. Click here for the list.
If you wish to use a card that’s on the restricted list for your chosen format, you may include only a single copy of that card, counting both your main deck and your sideboard. Currently, only the Vintage format uses a restricted list.