Online social networks are groups of actors formed by computer-mediated social interaction. These interactions are capable of establishing new social forms of groups and communities. Based on a discussion over several concepts of community and virtual community we propose the virtual community as a specific form of online social network. This theoretical debate is brought to the field studying the system named Fotolog during 2005 and 2006. Fotolog (www.fotolog.com) is a web service that allows for its users to post photographs or images with an associated text and other users may comment on each other’s posts. From the collected data, we propose a typology for communities found in these networks, based on their structure (network) and composition (social ties and social capital). We define three types of communities as associative virtual communities, emergent virtual communities and hybrid virtual communities.