Roblox is a massively multiplayer online video game and game creation system that allows users to design their own games and play a wide variety of different types of games created by other users. The platform hosts user-created games and virtual worlds covering a wide variety of genres, from traditional racing and role-playing games to simulations and obstacle courses. As of August 2019, Roblox has over 100 million monthly active users.
Cash shop sells Robux which you can use to buy cosmetics
Overall it's not really a game, more of an online social sandbox meant to share creations