Next.js is a versatile open-source development framework built on Node.js, initially released in 2016, that enhances React-based web applications by enabling server-side rendering and static site generation. It offers a robust developer experience with features like TypeScript support, smart bundling, route prefetching, fast refresh, and built-in CSS support, all without requiring special configurations. Server-side rendering, a method where HTML is rendered from the server to the client, helps improve page load times and SEO by making content immediately available for both users and search engine crawlers. Next.js integrates seamlessly with Express, a popular Node.js web application framework, allowing for advanced server-side rendering, easy creation of API routes, and backend capabilities like file uploading and data fetching before page rendering. While Express enhances Next.js's capabilities, a custom server might be preferred for specific infrastructure needs, although it may lack some of Next.js's built-in optimizations.