Word of Mouth (WOM) marketing is one of the most powerful forms of marketing. WOM consists of customer recommendations that influence sales. It is a trusted source of information. Opinion leaders, who are knowledgeable and sociable, informally influence product adoption through WOM. Various methods can measure opinion leadership, including asking people who they seek advice from. WOM is effectively used in social media and online communities. The diffusion of innovations theory explains how new products and ideas spread through communication over time within a social system. People have different rates of adoption, from innovators to laggards. Product characteristics like relative advantage and compatibility impact diffusion rates.