Sealed Deck or Sealed Play is a game type used in contests that involves limiting the tools and powers available to each player in a controlled fashion. In theory, this forces players onto a level playing field in which no one has a particular advantage or disadvantage over the other; the game play and style of each will be affected by the limits built in to the Sealed Deck. The name derives from sealing an actual deck of cards before beginning a game, e.g. for Magic: The Gathering's "limited" play.
Originally, ArenaNet offered Sealed Play as a card competition with actual cards at game conventions and other venues, although these tournaments are no longer held now. The printed cards hold the common skill information (such as cost and activation time), a description, and an image that's often a high resolution version of the game's skill icon. Players get a selection of skill cards, possibly chosen at random, from which they create their builds. The rules and printable card images are still available on guildwars.com, in case you want to play with friends.
In an October 2009 update, ArenaNet introduced the Codex Arena into Guild Wars. Similar to Sealed Play, it limits the skills to choose from (the "deck") on a 6-hour rota, requiring each player to create a build for their profession within those limits before engaging in PvP..