'React' is a declarative, efficient, and adaptable JavaScript library utilized for crafting user interfaces. It enables developers to build complex UIs by piecing together small, independent units of code known as 'components'. Originating from Facebook, React has swiftly risen to prominence within the web development arena.

Launched in 2013, a key feature of React is its virtual DOM, which significantly enhances rendering efficiency and overall application performance. This functionality allows the development of expansive applications capable of updating data dynamically without page reloads.

The component-based design of React introduces a modular approach, aiding in the creation of reusable UI components. This modularity not only elevates maintainability but also streamlines the testing process. Additionally, React's unidirectional data flow simplifies debugging, making it more predictable and manageable.

React is often integrated with other libraries and frameworks, such as Redux for state management, underscoring its versatility and compatibility with various backend technologies. This adaptability makes React a comprehensive solution for front-end development.

In conclusion, React's blend of simplicity, efficiency, and scalability, coupled with its extensive ecosystem and robust community backing, positions it as a preferred library for the development of dynamic and responsive web applications.