Also called "cyberterrorism" and "cyberwarfare," it refers to creating havoc by disrupting the computers that manage stock exchanges, power grids, air traffic control, telecommunications and defense systems. The traditional viruses, Trojans and denial of service attacks are part of the arsenal, all aimed at disrupting a government rather than a company. See virus
and denial of service attack
Information warfare is increasingly the first offensive move before the start of a physical attack. The military in many countries have full-time cyberwarriors on the payroll, because the more successful a cyberattack on an early warning defense system, the greater the success of the real attack. For example, according to the book "Cyber War," North Korea may have as many as a thousand hackers stationed in China, working on knocking out systems in South Korea and other countries.
The first book to deal with the subject was "Information Warfare: Chaos on the Electronic Superhighway," written by Winn Schwartau in 1994. Cited above, "Cyber War," by Richard A. Clarke and Robert K. Knake is an eye-opening treatise on the subject, released in 2010.