Vue.js, commonly referred to as Vue, is an open-source JavaScript framework used for building user interfaces and single-page applications (SPAs). Vue is designed to be approachable, flexible, and highly customizable, making it a popular choice among developers for creating dynamic and interactive web applications.

Key features and concepts of Vue.js include:

1. Component-Based Architecture: Similar to React, Vue.js encourages building applications using reusable components. Components encapsulate both the UI and behavior, making it easier to manage and maintain complex user interfaces.

2. Reactivity: Vue introduces a reactivity system that automatically updates the DOM whenever the underlying data changes. This system allows developers to declare data dependencies and efficiently manage UI updates.

3. Vue Directives: Vue provides special directives that allow you to bind data to the DOM, manipulate the DOM, and apply conditional rendering. For example, the `v-bind` directive is used to bind data to HTML attributes, and the `v-if` directive is used for conditional rendering.

4. Templates: Vue supports an HTML-based template syntax that allows you to declare the structure of your UI components in a declarative manner. These templates are then compiled into render functions for efficient rendering.

5. Vue Router: Vue Router is an official routing library for Vue.js. It provides tools for creating single-page applications with client-side routing, allowing you to define different views and navigate between them without full page reloads.

6. Vuex: Vuex is Vue's official state management library. It helps manage centralized state across your application and provides a way to share data between components.

7. Vue CLI: The Vue Command Line Interface (Vue CLI) is a tool that simplifies the setup and development of Vue.js applications. It provides a set of commands and configurations for creating, building, and deploying Vue projects.

8. Vue Devtools: Vue offers a browser extension called Vue Devtools that enhances the development experience by providing real-time debugging, component inspection, and state exploration capabilities.

Vue.js has gained a strong following in the web development community due to its gentle learning curve, clear documentation, and a balance between flexibility and structure. It can be used for both small projects and large-scale applications, and it's often compared to other popular frameworks like React and Angular.