Origins
Next.js is a framework built on top of React. You might be asking, Why is there a framework built on top of another framework? Well, as amazing as React is for helping us build applications, it's intentionally missing some things. Things like routing, styling, tooling, and SSR, to name a few. The community has stepped in to create awesome packages for us to build our own frameworks on top of React.
After years of building apps with React, leading opinions and conventions start to form in the community. You can think if Next.js as a framework that incorporates these ideas. The need to install anything on the framework level is almost nonexistent with Next.js. It's also a full-stack framework, allowing us to build out server-side logic and APIs.
When should you use Next.js?
Next.js is very flexible, with its many different rendering modes, making it ideal for many scenarios. I love to use Next.js for any React-based site/app I am working on for the web. You wouldn't use Next.js to make a component library or package, as it's designed to help you build applications.