Express is a minimal, unopinionated web application framework for Node.js. It sits on top of the built-in HTTP server and gives developers a clean way to define routes, parse requests, send responses and compose middleware functions, which makes it a common foundation for REST APIs, server-rendered pages and backend services of many sizes.
The official site documents installation, the application and router APIs, route parameters, middleware ordering, template engine integration, static file serving, error handling, cookie and session handling, and security and performance best practices for production deployments. A resources section points to guides, community middleware and related projects.
It suits backend engineers building JSON APIs for web or mobile clients, teams prototyping a service quickly, students learning how HTTP servers and middleware chains work, and organizations maintaining existing Node.js applications where a thin, well understood layer is preferable to a heavily opinionated framework.
Compared with full-stack frameworks it deliberately leaves database access, authentication and project structure to the developer, so setup is faster but conventions must be chosen explicitly; compared with the raw Node HTTP module it removes substantial boilerplate while keeping overhead low and the request lifecycle easy to trace.
Keep middleware order in mind because each handler decides whether to call next, always add a final error handler, validate and sanitize user input before it reaches business logic, and pin the major version since behavior and helper APIs have changed between releases.