Is multiuser operating system a multitasking operating system?

Nope it's not, a multiuser OS can be used by many people, one at at time. Lets say you have one computer for the whole family to share, you can use a different login for them so they can only log into their part only after you are done with it.

With a Multitasking OS you can do more then one thing at a time, like, watch a movie on your device and play a game at the same time.

Here's another analogy, rub your belly and at the same time pat your head, that's multitasking.