ASP.NET is a framework developed by Microsoft for building web applications. It allows developers to create dynamic websites and applications by providing a set of tools, technologies, and libraries.

Here are the steps to build a web application with ASP.NET:

1. Set up your development environment: Install the necessary tools, such as Visual Studio, .NET framework, and any other dependencies required for your specific project.

2. Create a new ASP.NET project: Open Visual Studio and select “New Project” from the File menu. Choose the ASP.NET template that best suits your needs, such as MVC (Model-View-Controller), Web Forms, or Core.

3. Design the user interface: Use HTML, CSS, and JavaScript to create the UI for your application. You can use Visual Studio’s drag and drop tools to design your pages or manually code them.

4. Define the data model: Determine the data structures and relationships needed for your application. You can use Entity Framework, a popular ORM (Object-Relational Mapping) tool, to simplify the creation and management of your data model.

5. Implement the business logic: Write the code that handles the application’s functionality, such as user authentication, data validation, and processing of user input. This code should be placed in the appropriate controllers, models, or service classes, depending on the chosen architecture.

6. Set up the database: If your application requires persistent data storage, you will need to create and configure a database. You can use a database management system like SQL Server or MySQL, and Entity Framework can help you interact with the database.

7. Test the application: Use unit testing and integration testing to ensure that your application functions as intended. Visual Studio provides built-in testing frameworks and tools for this purpose.

8. Deploy the application: Once your application is thoroughly tested and ready for production, deploy it to a web server. You can use various deployment methods, such as publishing directly from Visual Studio, using FTP, or using cloud-based hosting services.

9. Monitor and maintain the application: Regularly monitor your application for errors, performance issues, and security vulnerabilities. Apply updates and patches as needed, and consider adding logging and monitoring tools to track and troubleshoot any issues that arise.

Overall, building a web application with ASP.NET involves designing the user interface, defining the data model, implementing the business logic, setting up the database, testing, deploying, and maintaining the application. ASP.NET provides a rich set of features and tools to simplify these tasks and help you create robust and scalable web applications.