.NET is a big generic term. But it’s basically Microsoft’s platform/framework for developing web, desktop and mobile apps.
There’s the ASP.NET family (starting from MVC 1.0 to MVC 5)… there’s also the .NET Framework (from 3.5 to 4.5.1, 4.6.x)
Now, we have the new/latest .NET Core 1.0 (with 2.0 Preview currently out). This is the “new” Microsoft where everything is open-sourced, and cross-platform (Windows, OSX, Linux). The same project/source code will run on all 3 platforms in the same way. (Previously, .NET is only hosted on Windows servers).
You can create .NET applications using C#, VB, C++, F# being the most common. All in all, 44 different languages are supported by .NET. A few sampling:
COBOL for Microsoft .NET.
Perl for Microsoft .NET.
Eiffel for Microsoft .NET.
Python for Microsoft .NET.
Pascal for Microsoft .NET.
Mercury for Microsoft .NET.
Mondrian for Microsoft .NET.
Oberon for Microsoft .NET.
Salford FTN95 (Fortran) for Microsoft .NET.
SmallTalk for Microsoft .NET.
Standard ML for Microsoft .NET.
Dyalog APL for Microsoft .NET.
How big is the .NET framework? .NET Standard 1.6 has 13,000 API calls.
The new .NET Standard 2.0 has 32,000+ API calls!!!
One advantage of .NET is you can debug and step into your source code while debugging… something that other languages cannot do (by others, I mean PHP, JS, scripting languages, etc)
Also, you can deploy your application to your production server without also bringing the source code to the same server. The source code stays in your dev machine (or within your company) and you have the option to not deploy it on the production server. i.e. safe and secure.