The paper presents the results of a genre analysis of an online encyclopedia - Wikipedia, whose form is completely shaped by the web. The aim of the analysis was to determine characteristic features of the encyclopedia and point out the ways in which it differs from other available online encyclopedias. Analysis covered structural and stylistic features of the articles. Structural analysis defined the characteristics of the visual side of the encyclopedia, graphics, constructional elements of the site as well as systemic mechanisms. Stylistic analysis verified the language of the encyclopedia, language of articles and means of communication with the users. Linguistic analysis verified the degrees of formality, lexis as well as the most frequent syntactic structures. The results show how users acting through mechanisms provided by the system can shape the features of the content in particular ways. The paper also includes a comparative analysis of the Polish version of Wikipedia with its English and German equivalents.