The short answer: HeroEngine is the tech that let’s you develop an online game. HeroCloud is the out-of-the-box solution that includes HeroEngine and everything else you need to launch your game online.
HeroCloud is a service that includes HeroEngine, hosting, and billing. It is designed primarily for independent developers as well as educators, corporate and government simulation builders, and other projects. By licensing HeroCloud your team accesses a full implementation of HeroEngine running somewhere in the Internet “cloud” so your team can access it anywhere and so you don’t need to worry about hosting, configuration management, or other annoying distractions from developing your game.