A good question. Depending on who you ask it the definition may change. A general idea is an operating system is the middle man between underlying hardware and the programs that run on top of it. But an operating system can be something as small as 8MB(http://kolibrios.org/en/index) to something as complex as Distributed Operating Systems running on cloud. So in this paper we will discuss linux as traditional operating system and discuss why its insufficient for IOT purposes, and what is the need of RTOS'es.
The concept of Operating System is quite old when in 1964 when IBM developed IBM System/360 Operating System. Before then the computers did not have operating systems the programs had to be run directly on the hardware. Since then Operating Systems have come a long way handling different range tasks depending on user requirements. Here are some some environment