Game theory structures an interactive context in a way that helps to find actions - strategies - with specific properties. It automatically follows that different economic, political, management and social contexts may have the same game theoretic structure and therefore is studied in a similar way. Building and solving are linked activities. The structure often 'talks a lot', in that it helps underline the specificities of a game. People structure with the aim of solving the game, so they eliminate all that is not necessary to find the solutions. This chapter highlights these facts by giving all the elements of the structure of a game. The chapter discuses two main representations of a game: the strategic form game and the extensive form game. It focuses on central concept of game theory, the notion of strategy: develops pure, mixed and behavioural strategies. The book discuses concept of information and way utilities are assigned to the different ways to play the game.