What is the relation between Time Sharing operating systems and Round Robin CPU scheduler? So i have asked this before, but i still didn't get my answer so I'll try to make my question more clear this time : 

Based on my O.S book, Time sharing operating systems switch between processes very fast using a time slice and it uses interrupts when the time slice ends and switches to another process, so isn't this the definition of round robin CPU scheduling algorithm? so are they saying Time sharing O.S uses Round Robin cpu scheduling or what? 

if not, then what is the relation? for example what happens if our O.S is time sharing and uses Round Robin cpu scheduling vs when it uses another algorithm like FCFS?
No.  A time-sharing computer switches between processes in some way or other.  A time-sharing computer with round-robin scheduling switches between processes in a particular way.  For instance, suppose there are three processes, A, B, and C.  A round-robin scheduler would switch between them in the order ABCABCABCABC etc.  Some other scheduler might switch between them in the order AABAABAACAABAABAAC (for instance).  Both are examples of time-sharing, but the latter is not a round-robin scheduler.

I suggest reading https://en.wikipedia.org/wiki/Scheduling_(computing).