JavaScript Style Sheets (JSS) is a way to apply styles to web elements using JavaScript. Instead of using traditional CSS files, JSS allows developers to define styles directly within their JavaScript code. This approach can make it easier to manage styles dynamically, especially in applications where styles need to change based on user interactions or data. JSS is often used in modern web frameworks like React and Vue.js, where components can have their styles defined alongside their logic. This integration helps maintain a clear relationship between the appearance and behavior of components, promoting better organization and reusability in web development.