In this paper, I will explain the main terms that are important in information system security. Nowadays, computer play a big role in the everyday life od every individual. Advances in technology over the last few decades have resulted in the development of computers and computer programs all over the world. The reason for this is the increased concentration of exchanges data, and therefore it is almost impossible for the computer is not infected with one od the viruses. The result can be the permanent loss of important data, damage to the information system and difficulty working on the computer. A computer virus is a program that acts similarly to a biological virus in humans (e.g., Covid19). They attack cells or programs and system and multiply themselves. They are most received for specific files and spread when that file is launched. Apart from within one, if can also be extended to other computers via a computer network or external storage. The types od viruses, their actions, and the most important thing, the protection of operating system, are described. They are used by a huge number of users, and have large amounts od information scattered in operating system. In this case, there is an increasing danger of false or unauthorized information being planted and its destruction. Currently, there is not a single operating system that is protected in its entirety. The need for protection arises in the case od sharing inputs such as: memory, programs, data, etc.