Although cyberwarfare generally refers to cyberattacks perpetrated by one nation-state on another, it can also describe attacks by terrorist groups or hacker groups aimed at furthering the goals of particular nations. It can be difficult to definitively attribute cyberattacks to a nation-state when those attacks are carried out by advanced persistent threat (APT) actors, but such attacks can often be linked to specific nations. While there are a number of examples of suspect cyberwarfare attacks in recent history, there has been no formal, agreed-upon definition for a cyber “act of war,” which experts generally agree would be a cyberattack that directly leads to loss of life.

Cyberwarfare can take many forms, including:

• viruses, computer worms and malware that can take down water supplies, transportation systems, power grids, critical infrastructure and military systems;
• denial-of-service (DoS) attacks, cybersecurity events that occur when attackers take action that prevents legitimate users from accessing targeted computer systems, devices or other network resources;
• hacking and theft of critical data from institutions, governments and businesses; and
• ransomware that holds computer systems hostage until the victims pay ransom.

Source:http://searchsecurity.techtarget.com/definition/cyberwarfare