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.
Banning a card is uncommon. Announcements for changes to the Banned & Restricted lists are done all together on the Monday after a new set’s Prerelease, to go into effect with that set’s release. This way, the formats you play change as infrequently as possible.