Old .NET Framework was a main, Windows only, branch of .NET.
At tag "v4.x", a new branch named 'Core' was created. Core was a multiplatform version of .NET.
There were three tags in the Core branch: v1, v2 & v3.
Then Core jumped from v3 to v5 and was merged back into the main branch. ".NET Core" has replaced ".NET Framework". Also, v5 dropped 'Core' from the name and was named just ".NET 5".
Development now happens on the main branch and has already reached tag v7. A new tag is created each year.