The aim of this thesis is to provide a complete and useful description of what a videogame is. It is a common assumption amongst Game Scholars that videogames are systems: a set of interrelated parts that interact in different ways. This work analyzes the precise structure of that system and how it works. Videogames are a complex, multi-layered system that includes not only game elements, but the specific technology and the player. The videogame system has compulsory and optional elements that can be combined to create a vast range of videogames. Story is not an obligatory part of the videogame system, although many of its elements present narrative possibilities that can affect the player’s experience. This experience is created by the interaction of the system elements, is the reason why the player plays and goes beyond the mere idea of “fun”. In short: videogames are a multi-layered system that creates an experience for the player. This thesis is addressed to both game scholars and game designers.