Express.js is a minimalist and flexible web application framework for Node.js that simplifies the process of building robust and scalable web applications and APIs. It provides a set of features and tools that make it easier to handle HTTP requests, manage routes, and implement middleware, streamlining the development of server-side applications.

Key concepts in Express.js include:

1. Routing: Express.js allows you to define routes for different HTTP methods (GET, POST, PUT, DELETE) and URL paths. Routing enables the creation of endpoints that handle specific requests and return responses. For example, you can set up routes to manage user profiles, process form submissions, or serve static files.

2. Middleware: Middleware functions in Express.js are used to process requests and responses at various stages of the request-response cycle. Middleware can perform tasks such as logging, authentication, parsing request bodies, and error handling. Middleware functions are executed in the order they are defined, providing a powerful way to extend and customize the application's behavior.

3. Request and Response Objects: Express.js provides req (request) and res (response) objects to interact with HTTP requests and responses. The req object contains information about the incoming request, such as query parameters, request body, and headers, while the res object is used to send responses back to the client, including status codes, headers, and body content.

4. Static Files: Express.js can serve static files, such as HTML, CSS, and JavaScript, using the built-in express.static middleware. This is useful for serving assets like images or stylesheets in web applications.

5. Templating Engines: Express.js supports various templating engines that allow you to generate dynamic HTML pages. Popular templating engines include EJS, Pug (formerly Jade), and Handlebars. Templating engines enable you to embed dynamic content and logic into HTML files.

6. Error Handling: Express.js provides a mechanism for handling errors through error-handling middleware. You can define custom error-handling middleware functions to catch and manage errors that occur during request processing, allowing you to send appropriate error responses to the client.

7. Body Parsing: To handle form submissions and JSON payloads, Express.js uses middleware like body-parser to parse incoming request bodies. This allows you to access and process data sent by the client in a structured format.

8. Router: Express.js offers the Router class to create modular and maintainable routing. Routers can be used to define routes for specific parts of the application, enabling you to split the application into smaller, more manageable modules.

9. Environment Variables: Express.js applications often use environment variables to manage configuration settings, such as database connections or API keys. Libraries like dotenv can be used to load environment variables from a .env file.

10. Integration with Databases: Express.js can be easily integrated with various databases, both SQL (e.g., PostgreSQL, MySQL) and NoSQL (e.g., MongoDB). Using database libraries or ORMs (Object-Relational Mappers), you can perform CRUD operations and manage data effectively.

Express.js simplifies server-side development by providing a clean and intuitive API for handling HTTP requests and managing application logic. Its flexibility and extensive middleware ecosystem make it a popular choice for building web applications, RESTful APIs, and real-time applications with Node.js.