A product evangelist is someone who promotes a product without financial interest. The product evangelist does this purely and simply because he or she believes in the product and wants others to share this belief. Most promotion of product evangelists is done through word of mouth. But of course bloggers and social media are also very important means by which product evangelists can spread their 'faith'. Product evangelists are often early adopters of a brand.
Companies with many product evangelists often have a strong brand with which people like to identify. Think of brands such as Apple, Nike, Ferrari or Harley Davidson.
Companies do a lot to keep their evangelists happy. Nowadays there is no longer a company where you cannot become a fan of on Facebook. Many brands also organize meetings where fans of the brand can come together.
The term product evangelist or evangelist marketing was devised by management guru Guy Kawasaki.