Advantages and disadvantages of rtos?

RTOS are well... realtime.
It means that it is possible to create processes with strict time constraints (like : never more than 5ms delay, the "never" is the important part).

The disadvantage is that it will try to meet these time constraints at all costs. Even if it means using less efficient algorithms and completely suspending less critical processes.

Because they are often designed for embedded systems, RTOS are usually simpler and smaller than non-RT desktop OSes.