Viral marketing leverages the trust we place in friends or social networks to distribute advertising messages quickly to a large number of people in a short amount of time, usually through email. Other modes of transmission include instant messaging programs and internet forms that send emails onto others.
Viral marketing tends to work best when it involves either some kind of humorous or outrageous content (for example, a funny video, or a controversial message), so that people forward it on to members of their social group, or email list.