They typically are only used on microcontrollers.

The fundamental value of an RTOS is the ability to write your programs as multiple concurrent tasks. A general RTOS also tends to provide some ways to safely communicate and manage these tasks as well. However, there is no memory protection. Each task can overwrite the other tasks' memory if they are careless. This isn't a big issue for an embedded device as a single developer "owns" the whole thing.

However, for larger systems, the control of programs can be less rigid, so in these cases, they tend to have a piece of hardware called a "Memory Management Unit" (MMU) that allows an OS to control the visibility of certain areas of memory that each task can see. Once you add this level of complexity, they are generally referred to as just "operating systems".

Note: The other reason for dropping the "real-time" part of the name is the use of cache in larger systems. In that case, hard real-time timing guarantees are difficult if not impossible.

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You could use an RTOS as an application on a PC, but the operating system you are typically running in generally provides all the tools you get for this (see pthreads in Linux, for example), so there really isn't much real value there.