ASP.NET is an open-source, server-side web development framework created by Microsoft for building dynamic web pages, applications, and services. It supports multiple programming languages such as C#, VB.NET, and J#, and integrates with the Common Language Runtime (CLR) for flexibility and performance. ASP.NET offers various development approaches, including Web Forms, MVC, and Web Pages, each catering to different application needs. While ASP.NET provides high performance, cross-platform support, and robust security features, it also has drawbacks like higher costs and documentation issues. Comprehensive testing of ASP.NET applications is crucial, utilizing tools like Visual Studio and platforms like LambdaTest for unit, integration, and cross-browser testing. Despite challenges, ASP.NET remains a powerful framework for enterprise-level applications, with ongoing support for testing innovations and strategies to enhance software quality.