React is a powerful JavaScript library that allows developers to build user interfaces with ease and efficiency. At its core, React revolves around the concept of components. Components are reusable pieces of code that manage their own state, meaning that they can handle their own data and respond to user inputs. This modular approach not only enhances code reusability but also simplifies the debugging process. Furthermore, React utilizes a virtual DOM, which optimizes rendering by updating only the parts of the user interface that have changed, making applications faster and more responsive.

For beginners, understanding props and state is essential when working with React. Props (short for properties) are read-only parameters that allow data to flow from parent components to child components, creating a unidirectional data flow that enhances readability and predictability in applications. On the other hand, state is mutable and allows components to manage their own local data. This fundamental distinction is crucial for developing dynamic applications. By mastering these core concepts, new developers can start building their own interactive UI components confidently.

When it comes to styling your React components, choosing the right library can make a significant difference in both development speed and user experience. Here are the top 5 libraries that you should consider for styling your React applications:

1. Styled-Components: This popular library utilizes tagged template literals to style your components, allowing you to write CSS directly within your JavaScript files.
2. Emotion: Known for its flexibility and performance, Emotion offers both a styled-components API and a low-level CSS-in-JS approach.
3. Material-UI: If you're looking for a rich library of pre-designed components, Material-UI provides a set of React components that implement Google's Material Design.
4. Tailwind CSS: A utility-first CSS framework that enables rapid UI development with pre-defined classes. It works seamlessly with React to create responsive designs.
5. Ant Design: This design system is great for enterprise applications, offering a comprehensive set of high-quality components that can be easily customized.

Creating responsive user interfaces with React involves utilizing a combination of flexible layouts, grids, and media queries to ensure your application adapts seamlessly to various screen sizes. To get started, it's crucial to employ CSS frameworks such as Bootstrap or Material-UI, which offer pre-designed responsive components. Additionally, React's built-in capabilities, such as the useEffect and useState hooks, can help manage state changes in response to screen size alterations, enhancing the overall user experience.

Moreover, developers can leverage React libraries, like React Router for efficient navigation and Styled Components for styling, to further enhance responsiveness. Implementing a mobile-first approach in your design process is also essential. Begin by designing for smaller screens and progressively enhance the layout for larger displays. This allows for a smoother transition between devices and improves performance. Remember to test your Responsive User Interfaces on various devices to ensure consistency and functionality across the board.