Vue.js is a progressive JavaScript framework used for building user interfaces. It is designed to be incrementally adoptable, which means you can use it for single-page applications as well as add it to parts of an existing app. Vue is known for its simplicity, reactivity system, and component-based architecture.

Vue, React, and Angular are all popular front-end frameworks, but they have different focuses. Vue is known for its gentle learning curve and simplicity, making it easy to get started quickly. React is focused on flexibility and has a large ecosystem, while Angular is a more complete solution with more built-in features but is often considered more complex to learn.

The Vue instance is the root of every Vue application. When you create a Vue instance with new Vue(), it links the Vue app to an HTML element on the page and provides options for data, methods, lifecycle hooks, and more:

Vue CLI is a command-line tool that helps you quickly set up and scaffold a Vue project with features like hot-reload, linting, and CSS preprocessors. It streamlines the development process and provides options for adding plugins and configurations.