Since the development of Web 2.0 (or social media) sites such as Facebook, Twitter, Blogger, and various forums and communities, online users from all over the world have been exposed to a completely new means of information sharing: electronic word of mouth (e-WOM). Due to the fact that it is a recent research phenomenon, its definition is difficult to be phrased and similar to traditional word-of-mouth, the theoretical framework of e-WOM is not very clear. This complex concept is linked to viral marketing, user generated content, stealth marketing, opinion sharing, decision making and other aspects. Considering the multiple areas that e-WOM has an influence on, this article presents an overview of what has been researched with respect to this subject. Using the findings of this paper, it may be possible to set up the base of a conceptual model to measure electronic word of mouth.