.NET is a free, cross-platform developer platform for building various applications, initially released in 2002 and evolved into .NET Core and unified under .NET 5. It includes a comprehensive architecture overview, installation guidance, programming basics, data handling with ADO.NET and Entity Framework, web app development with ASP.NET Core, deployment strategies, security measures, performance tuning, and best practices. Additionally, the document includes a case study to illustrate real-world .NET implementation challenges and solutions.