Kill -9 is a command on Unix and Linux to send signal 9 (SIGKILL) to a process. That’s the version of kill that is the most reliable and has immediate effect.
Taskkill is a Windows command line program. I believe that taskkill /f uses the TerminateProcess() API. This is more forceful than the End Task button on the Task Manager. There is a different End Process button on the Task Manager that does use TerminateProcess().
Compare with the yearly release cycle on cars.