React is a JavaScript library focused on building reusable, encapsulated components for user interfaces, promoting a declarative programming style and efficient data flow through features like the virtual DOM and JSX. It supports component-oriented architecture which extends to mobile platforms such as iOS and Android via React Native. While it offers numerous advantages like simplified UI structure and component reusability, it is not a complete framework and can be verbose, making component writing less straightforward compared to pure HTML and JavaScript.