process thread task
I unix In UNIX (GPOS) all external commands become process, so in UNIX everything related to process. Where as in vxworks(RTOS) every thing related task.
Process is an instance of running programs. Thread is an smallest unit of execution.
Whenever u are creating process text segment, data segment, stack segment are created. But if u create thread instead of creating these three it will create only stack segment and share text segment and data segment. That is why we are saying thread is smallest unit of execution.