Structure of OS isn't precisely determined but we'll accept sturcture offered by some of highest authorities from OS theory area (Denis Richi, Per Hansen, Ken Tomson). According to them structure of Operating Systems is as follows:

1. Kernel (core of Operating Systems)
is representing vital part of Operating Systems from which key attributes are depending on: stability, reliability and efficiency. Every mistake on kernel files usualy means crash of Operating Systems and reqest for reinstalling.

2. Drivers
This part of OS ensures support for new pluged in hardware and secures isolation of kernel and application software from connected hardware, which is very impotrant because, development of Operating Systems is made easier.

3. Shall
This part connects every other parts into one union and synhronizes common work. Special part of shell presents user interface which directly affect simplicity and efficiency of using Operating Systems. Today it is usual, so called GUI (Graphic User Interface)

4. User tools and programms (utility)
This part of OS isn't necessary but it can make work confortable. In this group we can put different desktop themes, screen savers, debuggers, word processors and other. Special part of utility enables optimization of OS but is very unstable because it can cause OS crash. This is usualy weakest documented part of OS.

Vital Operating Systems caracteristics
On the market there are relativly big number of Operating Systems made for different categories of users. Every Operating Systems cann't be compared in different ways, but some caracteristics are in real life, more important then others:

Most important caracteristics of Operating Systems are:
1. Stability and reliability - presents average time between crashes of 2 Operating Systems in different conditions. These are extremly important datas because they can produce big financial damages. Server Operating Systems must have much better stability and reliability then Operating Systems made for individuals. Crash of server Operating Systems can led to stopping of funcioning of computer network with hudereds computers and cause big financial damages.

2. Functionality - presents rate of hardware support and support of application software to new hardware and software products. In real life Operating Systems made for individuals users show big rate of functionality and easier accept new hardware and software. Server Operating Systems usualy are much more conversative and accept to work only with products that are in their HCL (Hardware compatibility list) for specific Operating Systems.

3. Efficency presents speed of Operating Systems making its functions. We can talk about eficient Operating Systems which making of system operations is almost optimal (access to mamory, hard drivers, etc..) but there are Operating Systems that aren't efficient as those ones.

4. Simplicity of use presents average time thet is needed for usual user to learn Operating Systems functioning. For example, we can say that windows is relativly easy to use (user friendly), while Unix-like Operating Systems are relativly hard for application, they require more learning time.

Operating Systems, in global we devide to network Operating Systems, and Operating Systems made for individuals. Network OS are those OS which are used in network enviroment, which means that they should be more stabil and reliable.

Most known, newer network Operating Systems are:

- Windows NT (Server; Work Station)
- Windows 2000 (Professional, Server, Advanced Server, Data Centar)
- Windows XP Pro
- Windos 2003 Server
- Unix
- Linux
- Novel NetWare

OS made for individuals can possess limited network ablilities, but that isn't their primary purpose. Most known representations are:

- DOS
- Windows 95/98/98 SE, ME, XP Home Edition