What is deck-building in games?
A mechanic in which acquiring cards during play is itself the main decision — you start with a weak, identical deck and construct a better one as the game progresses, and your deck's improving quality is the engine of your progress.
How it works. Every player begins with the same small deck of basic cards. On each turn you draw a hand, play it, and use the resources it generates to buy new cards from a shared market. Purchased cards go into your discard pile, and when your draw pile runs out, the discard is shuffled to form a new one — so everything you buy eventually comes round.
Why that shuffle mechanism matters. It creates the central tension: buying more cards makes your deck stronger and more diluted. A powerful card appears less often in a large deck. Good play therefore means being selective, not acquisitive — which is counterintuitive, since buying things feels like progress.
The key strategic concepts:
Deck thinning. Removing weak starting cards is frequently more valuable than adding strong ones, because it increases the frequency with which your good cards appear. Cards that let you trash other cards are consequently prized.
Engine building. Combining cards that draw more cards and generate more actions so that a single turn cascades into many plays — the characteristic satisfying moment of the genre.
Opportunity cost. Every purchase is a permanent commitment to a deck you cannot easily change.
Where it came from. Dominion, published in 2008, established the mechanic and created the genre. It was influenced by collectible card games, but removed the collecting — all cards are available to all players from the start, so it is a game of decisions rather than of purchasing power.
Where it went. The mechanic spread widely and has been combined with almost everything — board movement, cooperative play, and legacy campaigns. It has become a staple of video games too, where roguelike deck-builders apply the same loop to single-player runs, and it is one of the clearest cases of a board game idea influencing digital design rather than the reverse.
Bag-building and dice-building apply the identical principle with different components.