Viral marketing refers to marketing techniques that use pre-existing social networks to spread awareness of a brand or product through self-replicating viral processes like the spread of viruses. It uses things like video clips, games, images, or text messages that people want to share with their social networks. The document discusses how viral marketing has evolved from using video repositories to content distribution across platforms like social media. It provides an example of how marketing an engagement metric was made viral through seeding it on social networks and blogs to generate interest before revealing its true source.