ating system is the system SOFTWARE that runs a computer or a computing device or gadget.Linux,  APPLE Macintosh, Unix, Xenix, Sun Solaris, Apple IOS, Palm OS, MS DOS, MS Windows ...MS Windows is mainly for interface to the end user - who may not be a computer educated or scientific person.  This is mainly for a small system , personal computer meant for one person.  It is mainly Graphical user interface BASED.  It is developed by Microsoft Inc.Unix, Linux, Xenix, Solaris are for those who are in educational environment, development environment.  These systems are used for deployment in customer sites hosting applications.  These are used as servers.  These can also be used for  development and graphical user interface purpose.  It was originally developed by AT & T Bell labs.Apple IOS or Iphone OS, is the operating system for mobile phones mainly meant for Apple Inc.'s hardware.  IOS is an operating system for Ipad and Ipod also.  The Wifi communication based tablet computers use this.Mac OS is the operating system (GUI based) for the Macintosh computer systems of Apple Inc.Linux is a free OS and sources are available.  It is an open source OS.  It is mainly used in educational environment.Unix based systems are supposed to be safer against viruses.  MS Windows systems are prone to attacks from virus.RTOS or real time operating systems which are mainly meant for real time applications.  The RTOS has to perform certain actions WITHIN some time (in milliseconds) of  receiving input signals.  They are used in embedded systems (modern gadgets) like a washing system for example.  They are used in AUTOMATED machines, devices , defense industry etc.  RTOS systems are used in Robots and scientific research.