The lesson "RETI e SegRETI" aims to introduce the information technology discipline of the information systems security. Every life phase of information, from communication, to memorization up to elaboration, requires to guarantee the data security, in other words every activity has to be the desired one and this has to happen even when events which could cause detrimental and/or dangerous behaviours happen. The lesson is focused on the security properties required to an information system (confidentiality, integrity, availability, authenticity, controlled access), in default of an information loses, completely or in part, its usefulness and on the mechanisms allowing to face intentional attacks aimed to damage the security properties. The analysis of security properties and countermeasures will be presented through some significant cases of ordinary use of information applications, such as social networks (Facebook, etc...), email, music files e-commerce.