Constructed is a term used to refer to decks which have been created using cards from the player's collection. Constructed play is play in either Play mode, Practice mode or Friendly Challenge. Some Tavern Brawls also use constructed decks, but due to the special rules and effects in play, are not usually referred to as constructed play.
The term is used to contrast with Arena play, where each player's deck is built from randomly selected cards. Constructed is the basic type of play for Hearthstone.
Constructed is Hearthstone's main type of play, found in Play mode, Practice mode and Friendly Challenge, as well as some Tavern Brawls. In constructed play, the player does battle with an opponent, each using decks constructed from their own collections. Only cards found in the player's collection can be included in their deck, providing motivation to earn new cards in order to be able to include them in decks. Each constructed deck can only feature a maximum of 2 of any one card, and only 1 of each legendary card.
In contrast, decks built in Arena, although also constructed by players prior to engaging in battle, are not drawn from the player's collection, instead being chosen one card at a time from random selections of possible cards, and have no limits on the number of duplicate cards allowed.
While both types of play feature a strong element of strategy in the building of the deck, constructed play offers a far more polished and perfectionistic approach, with players able to tailor their decks to their precise preferences, and adjust them as often as desired. Once built, Arena decks cannot be changed, and once an Arena run is ended, cannot be used again.