Gatherer is the Magic Card Database. Search for the perfect addition to your deck. Browse through cards from Magic's entire history. See cards from the most recent sets and discover what players just like you are saying about them.
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.