Authentication in React and Next.js involves securing user data and access to protected components, with React being a client-side library focusing on dynamic interfaces and Next.js serving as a framework enhancing React with production-optimized features. React relies on third-party tools for functionalities like routing, whereas Next.js offers built-in options such as server-side rendering and static site generation. Implementing authentication in React typically requires a server, client-side components, and tools like React-router, while Next.js simplifies the process with NextAuth.js, leveraging API routes for integrated authentication logic. Clerk provides a more streamlined solution for both frameworks by offering dedicated libraries and components that simplify the authentication process, making it straightforward to implement essential features like login and logout. Clerk's integration is particularly suited for developers looking for efficient and secure authentication management without delving into the complexities of traditional setups.