Sometimes it's difficult to articulate word of mouth in a formal context. But this helps... According to a recent post by McKinsey, there are three types of word of mouth: Experiential, Consequential and Intentional.
According to the firm, Consequential is when people respond to a marketing idea and pass it along publicizing a brand. Intentional is when marketers specifically enlist celebrities or endorsements. The former is much more common than the latter as we pass around interesting stuff all the time.
But the most common form of word of mouth (both online and offline) is Experiential. This is someone's direct experience with a product or service, and mostly when that experience deviates from what's expected. McKinsey estimates that between 50 - 80% of word of mouth in any product category is, in fact, Experiential.
50 - 80%... Win the experience, win word of mouth.