This is the classic Magic booster pack. You know what you're getting here: 10 commons, 3 uncommons, 1 rare or mythic rare, 1 land, and 1 token/ad card. Around a third of the time, one of the commons will be replaced by a foil of any rarity. It's also possible that you get a card in a showcase frame, an exciting alternate version of a normal M21 card.
Additionally, M21 Draft Boosters occasionally contain borderless cards!
A boosterbox contains 36 boosters.