Two years ago, .NET in an everyday connotation would imply C# (VB.NET is mainly only used in legacy corporate environments, although there was nothing stopping people from implementing greenfield VB projects until last year or so) and an installation of the .NET Framework (equivalent to Java runtime environment) on a Windows PC.
Today, it is increasingly referring to .NET Core, which is still likely C# (with a smattering of F# from enthusiasts) but without a runtime necessarily pre-installed on the PC, now available as first-class citizens (and developed in concert with) on Windows 7+, macOS, Linux, and soon FreeBSD. The runtime has been broken down from the monolithic framework to hundreds of individual libraries (all available via a package manager) compiled to platform- and architecture-independent dlls available via the binary nuget package manager.
Back when .NET was first getting started there were a lot more languages (Microsoft paid language developers to port Java (the language) and the community provided ports of Python, Ruby, and others; while new languages specifically designed for .NET also came and went; I have somewhat fond memories of learning Boo, but I don't think it saw any updates this side of .NET Core 1.0), today it's mostly just C# and F#.
More interestingly, there were some serious attempts at introducing native MSIL (Microsoft Intermediate Language) the assembly-level language interpreted by the runtime environment, now known as CIL or Common Instruction Language) in the form of microcontrollers that natively executed MSIL instructions, but besides a few extremely niche implementations (and very expensive - I remember being disheartened at the time) that ended up going nowhere and I'm not aware of any modern efforts at revisiting that, although I wouldn't be surprised given the resurgence of .NET in recent years.
Today, .NET Core is being pushed for all desktop, mobile, and web development; and is supported on major consumer platforms. The entirety of .NET Core development is out in the open (and there are now official public committees for furthering its development and making decisions affecting its future) and the code is actually available on GitHub.
I jumped on board the .NET train when I found a letter to the only tech-literate teacher at my high school who had thrown it away; it offered to send a free sample pack of CDs and basic literature in advance of the release to interested schools back in 2001 or so (when J# was still a thing, C# had just been introduced, .NET was still at the "we don't speak of it" 1.0 mile marker before the hard fork to redo the CLR with support for generics and revisit some poor decisions in the initial release, and the "new" ASP.NET offering still used WYSIWYG to design the layout!) and managed to get them to send me a copy. It feels like forever ago! C# stagnated for some years, but then Microsoft became serious about it once again after the Windows Vista release (and after they failed to port the Windows userland to .NET due to serious performance constraints in particular pertaining to GC with the Longhorn project), but then saw some great updates that made it an incredibly well-designed and efficient language (without even taking the standard library into account).