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Diablo III Wiki
Seasons are Diablo 3's version of the Diablo 2 Ladders, and they were added to Reaper of Souls in Patch 2.1 in August 2014. When a season is active, players can make new characters (hardcore or softcore) and click the "Seasonal Hero" button during creation. That will designate their characters as Seasonal, and they will only be able to play or interact with other Seasonal Characters for the duration of that season.
Each Season is a fresh start; players can not bring in any of their existing characters, items, paragon points, gold, artisan levels, etc, from their normal account. This puts everyone on a level playing field, and Blizzard sees it as a way to bring players back into the game, as they can play with others of their power level, rather than trying to join public games with Paragon 700 Torment 6 steamrollers.
Special season-exclusive content includes Leaderboards, Conquests, and Seasonal Legendaries.
Seasons will run for a TBD time, presumably around 3 months, and when a season ends all gold, items, characters, etc are merged back into a player's main, non-seasonal account.[1] As each season ends a new season will begin, and if players want to take part they must create new seasonal characters and go from scratch all over again.