Word of mouth advertising (WOM) involves unpaid positive marketing messages spread from person to person. It can be transmitted through direct human communication or online. WOM is a powerful promotional tool that should be part of every marketing strategy due to its ability to influence consumer purchasing decisions. The document discusses various types of WOM advertising, strategies to encourage positive WOM, and disadvantages like the inability to track results. It also provides examples of viral marketing campaigns and how brands have leveraged WOM through social media.