Vanguard is a supplement and a card type for Magic: The Gathering that debuted in Arena's Summer Season of 1997 and are used in the format of the same name.
The oversized (3" × 5") Vanguard cards aren't played like regular cards. Instead, each player chooses one of the character cards before the match begins and plays under the conditions set by that card.
A Vanguard card modifies your starting (and maximum) hand size, as well as your starting life total. It also contains one or more abilities. Sometimes they're abilities that you can play; sometimes they're abilities that just change aspects of the game. Any abilities printed on a Vanguard card work exactly like those of an in-play Magic card. Note, however, that Vanguard cards are colorless, aren't permanents, and will remain in the command zone the whole game, so they can't be affected by spells or abilities.