As a popular culture in the twenty-first century, online gaming has largely increased the scope of video game culture, which has attracted game players from a range of ages and occupations around the world. This book reviews the brief history of online games and how computer technologies facilitate online game development. The first online game was PLATO (Programmed Logic for Automatic Teaching Operations) designed by University of Illinois in 1969. Since that, online game has been developed to four generation. This book emphasizes the types and characteristics of game through introducing various games. Online game can be understood as a type of game that composites different game types such as FPS (First Person Shooting game), MMORPG (Massive Multiplayer Online Role-Playing Game), Simulation games, Actions games and Sport games. The author tries to analysis the online game from play, pretending, goal and rules that are fundamental for a game design. It not only provides the discussion of online game stories but also outlines the roles in a game. It hopes that this book can bring game players, educators and researchers a new viewpoint to further understand online games.