An Operating System is the basic software of computers that provides an interface between the computer programs and hardwares.
Operating system also provides a software platform on top to other programs, they are called application programs (Application program: is the software that helps the users to make typical functions, like making text, others). Your choice of the operating system, determinates the great extent of the application that you can run.
The basic functions of the O.S. are managing machine resources, coordinate the hardware and organize files and directories on storage devices.
Classification of Operating Systems
Operating systems can be classified as follows:
Multi-user: is the one that concede two or more users to use their programs at the same time. Some of O.S permits hundreds or even thousands of users simultaneously.
Single-User: just allows one user to use the programs at one time.
Multiprocessor: Supports opening the same program more than just in one CPU.
Multitasking: Allows multiple programs running at the same time.
Single-tasking: Allows different parts of a single program running at any one time.
Real time: Responds to input instantly. Operating systems such as DOS and UNIX, do not work in real time.
Operating system functions
The operating system serves several functions:

Operating System Processor manages the distribution among programs using a programming algorithm.

Management Random Access Memory:

Operating system manages the memory space allocated for each application and each user, if appropriate. When physical memory is insufficient, the O.S creates an area of memory on the hard drive, called “virtual memory.” Virtual memory permits you to run applications that require a capacity of memory beyond available RAM in the system. However, this memory is much slower.

Management of input / output:

Operating system to unify and control access to material resources programs through the drivers (also known as administrators peripheral or input / output).

Execution Management applications:

Operating system ensures that applications run smoothly by allocating the resources they need to function. This means that if an application does not respond properly may “succumb”.

Operating system is responsible for security in connection with the execution of programs by guarantee you that resources are used only for programs and users with appropriate authorization.

The O.S manages all the writing and reading in the file system and access permissions to files and user applications. File system that permits files to be recorded in a tree structure.

Operating system provides hundreds of indicators that can be used to diagnose the operation of the equipment.
Operating System Components
The system consists of a set of software that can be used to manage interactions with the hardware. These items are usually included in this set of software:
The core: This represents the core operating system functions, such as memory management, processes, files, inputs / main outputs and communication functions.
The shell: This enables communication with the operating system through a control language, allowing the user to control the device without knowing the characteristics of hardware, the management of physical addresses, and so on.
First operating system was developed by IBM with a young man named Bill Gates, this could run on different computers from different manufacturers, it was called DOS (Disk Operating System), but differences between the parties did not set off a pitch. DOS was just a text screen with a command line that tells us which directory as we were only data for guidance. You had to “know” that “things” had to write for the machine to “do something. There wasn’t a context menu, and graphical displays to guide us.
But in the beginning there were these systems presented so elegantly with many colors, there was only the command line interface that to only people who had great computer knowledge could use the computers.
OPERATING SYSTEM IN THE 80s
At 80’s appear Mac OS systems and MS-DOS, Windows.
The exponential growth of users, most of them without any knowledge of languages for high or low, made in the 80, the priority of designing an operating system was the ease of use, thus resulting the first user interfaces.
Macintosh is the name under which we currently refer to any personal computer designed, developed, built and marketed by Apple Inc. Macintosh 128K was released on July 22, 1984 and was the first personal computer that was successfully marketed, which used a GUI and mouse instead of the standard of that time, the command line interface
GUI makes use of a WIMP environment (windows, icons, menus and pointer). The background of the screen is called desktop, which contents an image called icons.
Apple in 1984 produced the Macintosh, the first computer with mouse and graphical user interface (GUI). A few years later, Microsoft launched Windows, another operating system based in graphics and intuitive tools
List of OS:

Windows 7
Windows XP Professional
Ubuntu
Macintosh OSX
Microsoft Vista
Fedora
Mac OS X Leopard
Microsoft Windows 1.0
Xandros Linux
Microsoft Windows 3.1
Unix
Linux

Microsoft Windows
Microsoft Windows is a series of software operating system based in graphical users interfaces produced by Microsoft.
The different versions of Windows are:

Windows 1.0
Windows 2.0
Windows 3.0
Windows 95
Windows 98
Windows XP
Windows Vista
Windows 7

Mac OS
Mac OS is an operating systems developed by Apple Computer Inc. Macintosh is popular because the graphical user interface, it was the integral and unnamed system software first introduced in 1984 but is usually it referred to simply as the system software.
Mac OS can be divided into two families:

The Mac OS Classic family.
The Mac OS X operating system.

UNIX
This was developed in 1969 by a group of employees of AT & T Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas Mcllroy, and Joe Ossanna. UNIX was developed in assembly language, but 1973 had been almost completely recoded in C, facilitating their development and migration to other hardware. This Operating System’s found on mainframes and workstations in corporate Installations.
Linux
Linux has its origin in UNIX. He showed in the sixties, developed by researchers Dennis Ritchie and Ken Thompson of AT & T Bell Labs.
Andrew Tanenbaum developed a Unix-like operating system called Minix to teach students to design an operating system. Due to the teaching approach of Minix, Tanenbaum never allowed him to be altered, and complications that could be introduced into the system for their students, but Finnish student named Linus Torvalds, verifying that it was not possible to extend Minix, decided to write his own operating system compatible with UNIX.
Linux can be installed on all kind of computer no matter the hardware. This O.S is a leading server operative system, and can runs the 10 fastest supercomputers in the entire world, and the best two thing of it is that u don´t get virus and is free.
Mobile Operating System
The mobile O.S is the Operating system that controls all mobile devices.
The different systems for mobiles are:

Windows Mobile
Palms OS
BlackBerry OS
Symbian OS
Android