博弈论中，与正则形式相应，扩展形式通过树来描述博弈。每个节点（称作决策节点）表示博弈进行中的每一个可能的状态。博弈从唯一的初始节点开始，通过由参与者决定的路径到达终端节点，此时博弈结束，参与者得到相应的收益。每个非终端节点只属于一个参与者；参与者在该节点选择其可能的行动，每个可能的行动通过边从该节点到达另一个节点。

和正则形式不同，扩展形式允许互动的显式模型（explicit modeling of interactions），互动中，一个参与者可以在博弈中多次行动，并且在不同的状态中可以做出不同的行为。

An extensive-form game is a specification of a game in game theory, allowing (as the name suggests) explicit representation of a number of important aspects, like the sequencing of players' possible moves, their choices at every decision point, the (possibly imperfect) information each player has about the other player's moves when he makes a decision, and his payoffs for all possible game outcomes. Extensive-form games also allow representation of incomplete information in the form of chance events encoded as "moves by nature".