The new generation of security threats has been promoted by digital currencies and real-time applications, where all users develop new ways to communicate on the Internet. Security has evolved in the need of privacy and anonymity for all users and his portable devices. New technologies in every field prove that users need security features integrated into their communication applications, parallel systems for mobile devices, internet, and identity management. This review presents the key concepts of the main areas in computer security and how it has evolved in the last years. This work focuses on cryptography, user authentication, denial of service attacks, intrusion detection and firewalls.