Online communities support collective action without many of the constraints that have belied collective actors and formal organizations in the past. They have become increasingly pervasive platforms for activism as well as potential catalysts for novelty in organizing practices. Scholars have shown that by leveraging affordances of the Internet, these communities have displaced or become complements to face-to-face organizations such as churches, community centers, labor unions and political groups that have traditionally structured civic engagement. Few empirical studies, however, systematically address how processes ranging from mobilization to the coordination of complex, large-scale collective action and practices that enable and support these processes are different in online environments. In this dissertation, I provide conceptual background that supports the study of online communities as dynamic and diverse modes of civic engagement. I reveal how locations, boundaries, interactions and identities are instantiated differently in online communities, influencing processes and practices that are crucial to social change. Using Internet-based ethnographic methods, I examine: (1) how an online community called `Anonymous' experiences shifts in purpose as it transitions from being focused on recreation to becoming both an incubator and support system for several social change projects and (2) how the community adopts a repertoire of coordinating practices that allows it to organize complex projects.