C# vs .NET
C# is not the only language we can run under .NET. In 99% of the cases it runs under .NET. .NET is a framework that enables running applications wrtitten in C#.
Some other .NET-compatible programming languages are F#, IronPython, VB etc.. The cool thing is that we can use the code written in one language in the code wrtitten in another. For example, we could have a C# class drived from a F# class. We can think of C# as airplane and .NET as airport.
.NET
Its most important part is the execution environment called Common Language Runtime, which is responsible for managing the memory, providing error handling, dealing with threads and more. .NET also provides a set of standard libraries, such as System.Linq, System.Collections and many more (those System libraries). 可以把.net理解为java世界里的JRE，which includes JVM（和CLR)很像，还有一些标准库。
.NET-related frameworks
Windows Forms / Windows Presentation Foundation (WPF): create desktop applications
ASP.NET MVC: web development
.NET MAUI: Cross-platform framework for creating native mobile and desktop apps.