A few decades ago, most computers were stand-alone machines: they were able to process information using their own resources. Later, computer systems were connected to each other enabling a computer system to exchange data with another computer and to use resources of another computer. With the coupling of computer systems, security items emerge: in general the link between computers is not protected by thorough physical means. That enables an attacker to tap or modify information on the link. In this article, a description of the architecture of a wired and a wireless computer network is given. For both types of networks, threats to the process of information exchange are indicated. Security services and security mechanisms are addressed aiming to secure the information exchange between computer systems.