best rtos books
RTOS as name suggest is a real time operating system. Apart from its real time nature, it functions as normal OS(Windows,Linux).
OS is a abstraction to the low level system functionality. Depends on the OS architecture you can create a task,process,thread.
Lets take a hypothetical example, say you have RTOS X, and it supports only one process(task), and supports n threads. Now you can write a program which uses the API provided by OS to perform some operation. Compile it, link it with RTOS and burn in on the hardware,
Now there is only one user process is running and thread you have created, But depends on the architecture of the OS, there may be some other system processes running to do memory managment, thread management, interrupt handling etc.