A set in Magic: The Gathering is a pool of cards released together and designed for the same play environment. Cards in a set can be obtained either randomly through booster packs, or in box sets that have a fixed selection of cards. An expansion symbol and, more recently, a three-character abbreviation is printed on each card to identify the set it belongs to.
Expansions are sets that feature the latest Standard-legal cards and mechanics. The most recently released expansions form a large part of the game’s Constructed and Limited environments. Each expansion has a theme running through the gameplay and flavor of its cards. Until 2018, expansions were organized into blocks according to their theme and release date. The newest expansion is Guilds of Ravnica.