google rtos anjoy
"A realtime system is one in which the correct operation of the system depends not only on the logical correctness of any computed result, but also on the time at which the result is delivered. A system in which all computed results must be delivered on time is a hard realtime (HRT) system. A system in which the computed results are sometimes allowed to be late is called a soft realtime (SRT) system." [Original source lost]
A true realtime OS is an HRT system; a system in which the designed-in capabilities of processor and OS are such that there is no case when the processor is in conflict by having to perform two tasks simultaneously. A "regular" O.S. could theoretically be an HRT system, but if the overhead in the system causes delay, or unexpected inputs must be delayed (held) in a buffer for processing, then the system has defaulted back to an SRT at best. Xp for example has tons of overhead which is necessary for advanced applications but is not necessary for a task-specific embedded controller.
A realtime system is necessary when there is no tolerance for fault or misstep and timing is critical. As an example, if a digitally controlled autopilot starts missing feedback values, and reads "old" data, uncontrolled oscillations could occur.
A realtime OS for a given system is any OS that is able to handle every command, input, or output, everytime, without failure, forever.