In a casual game, a card you choose from outside the game comes from your personal collection. In a tournament event, a card you choose from outside the game must come from your sideboard, which includes an arbitrary number of basic lands. You may look at your sideboard at any time.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.