classnames is a simple JavaScript utility for conditionally joining class names together. It's a fundamental tool for React, Vue.js, and other front-end projects where dynamic class manipulation is common. The library takes any number of arguments which can be a string or object and returns a single string of class names. It's lightweight, dependency-free, and handles truthy/falsy values gracefully, making it easy to manage CSS classes in a declarative and maintainable way.