The Loader specification (whatwg.github.io/loader/) is a work in progress specification that "describes the behavior of loading JavaScript modules from a JavaScript host environment". In other words, it describes a standard way to load JavaScript modules in both the browser and the server. It is developed by the WHATWG but not yet adopted as a living standard.
SystemJS is an implementation of the Loader specification that works on the browser. More specifically, SystemJS is a universal dynamic module loader. Here, "universal" means it can load not only CommonJS modules, but also ES6 modules, AMD, and global scripts. It does this through the SystemJS.import method, which is akin to a universal require ...
