There used to be .NET Framework (which was a Windows only runtime built by Microsoft) and Mono (an open source implementation for various other platforms).
In 2016 they started building .NET Core which is new open source implementation (built mostly by Microsoft) which runs on more platforms. For a while all three existed side by side.
Eventually .NET Core caught up and overtook the other implementations. These days Framework is legacy. Core has been renamed to just .NET (since it's now _the_ runtime) and Mono (as far as I know) has been totally replaced by it.
This is all from memory, so apologies for any inaccuracies!
In 2016 they started building .NET Core which is new open source implementation (built mostly by Microsoft) which runs on more platforms. For a while all three existed side by side.
Eventually .NET Core caught up and overtook the other implementations. These days Framework is legacy. Core has been renamed to just .NET (since it's now _the_ runtime) and Mono (as far as I know) has been totally replaced by it.
This is all from memory, so apologies for any inaccuracies!