The operating system underpins the entire operation of the modern computer.
Abstraction of hardware
The fundamental operation of the operating system (OS) is to abstract the hardware to the programmer and user. The operating system provides generic interfaces to services provided by the underlying hardware.
In a world without operating systems, every programmer would need to know the most intimate details of the underlying hardware to get anything to run. Worse still, their programs would not run on other hardware, even if that hardware has only slight differences.
We expect modern computers to do many different things at once, and we need some way to arbitrate between all the different programs running on the system. It is the operating systems job to allow this to happen seamlessly.
The operating system is responsible for resource management within the system. Many tasks will be competing for the resources of the system as it runs, including processor time, memory, disk and user input. The job of the operating system is to arbitrate these resources to the multiple tasks and allow them access in an orderly fashion. You have probably experienced when this fails as it usually ends up with your computer crashing (the famous "blue screen of death" for example).
Programmers want to write programs that will run on as many different hardware platforms as possible. By having operating system support for standardised interfaces, programmers can get this functionality.
For example, if the function to open a file on one system is
open(), on another is
open_file() and on yet another
openf() programmers will have the dual problem of having to remember what each system does and their programs will not work on multiple systems.
The Portable Operating System Interface (POSIX) is a very important standard implemented by UNIX type operating systems. Microsoft Windows has similar proprietary standards.
On multi-user systems, security is very important. As the arbitrator of access to the system the operating system is responsible for ensuring that only those with the correct permissions can access resources.
For example if a file is owned by one user, another user should not be allowed to open and read it. However there also need to be mechanisms to share that file safely between the users should they want it.
Operating systems are large and complex programs, and often security issues will be found. Often a virus or worm will take advantage of these bugs to access resources it should not be allowed to, such as your files or network connection; to fight them you must install patches or updates provided by your operating system vendor.
As the operating system provides so many services to the computer, its performance is critical. Many parts of the operating system run extremely frequently, so even an overhead of just a few processor cycles can add up to a big decrease in overall system performance.
The operating system needs to exploit the features of the underlying hardware to make sure it is getting the best possible performance for the operations, and consequently systems programmers need to understand the intimate details of the architecture they are building for.
In many cases the systems programmers job is about deciding on policies for the system. Often the case that the side effects of making one part of the operating system run faster will make another part run slower or less efficiently. Systems programmers need to understand all these trade offs when they are building their operating system.
 The X comes from Unix, from which the standard grew. Today, POSIX is the same thing as the Single UNIX Specification Version 3 or ISO/IEC 9945:2002. This is a free standard, available online.
Once upon a time, the Single UNIX specification and the POSIX Standards were separate entities. The Single UNIX specification was released by a consortium called the "Open Group", and was freely available as per their requirements. The latest version is The Single Unix Specification Version 3.
The IEEE POSIX standards were released as IEEE Std 1003.[insert various years, revisions here], and were not freely available. The latest version is IEEE 1003.1-2001 and is equivalent to the Single Unix Specification Version 3.
Thus finally the two separate standards were merged into what is known as the Single UNIX Specification Version 3, which is also standardised by the ISO under ISO/IEC 9945:2002. This happened early in 2002. So when people talk about POSIX, SUS3 or ISO/IEC 9945:2002 they all mean the same thing!