Vue.js is a progressive JavaScript framework for building user interfaces. It excels at reactive data binding (data changes automatically update the UI) and component composition. Vue 3 introduced the Composition API, bringing functional programming patterns similar to React Hooks. Vue is known for developer experience: intuitive API, excellent documentation, and fast learning curve. It's widely used in startups, mid-market companies, and enterprises. The ecosystem is mature: routing (Vue Router), state management (Pinia), full-stack framework (Nuxt), and testing (Vitest) are all excellent.