JSS stands for JavaScript Style Sheets, which is an authoring tool for CSS that allows developers to use JavaScript to describe styles in a declarative, conflict-free, and reusable way. It can compile in the browser, server-side, or at build time in Node.js. JSS does not extend front-end frameworks or change their way of working but uses framework tooling and capabilities to provide ready-to-use functionality, such as custom routing solutions.

Sitecore JSS (now called Sitecore Headless) is a framework for developing JavaScript applications that can be used to build custom web-based UI components and plugins. It also helps developers create reusable modules in Sitecore content management system (CMS) . The framework provides an API that makes it easy to use the latest version of React, as well as other libraries like D3, Bootstrap, jQuery, Axios, and Lodash. Developers can use Sitecore JSS with any client-side technology or library they want.

In summary, JSS is a tool that allows developers to use JavaScript to describe styles in a declarative, conflict-free, and reusable way, while Sitecore JSS is a framework for developing JavaScript applications that can be used to build custom web-based UI components and plugins and create reusable modules in Sitecore CMS.