More and more weakly structured, and irregular data sources are becoming available every day. The schema of these sources is useful for a number of tasks, such as query answering, exploration and summarization. However, although semantic web data might contain schema information, in many cases this is completely missing or partially defined. In this paper, we present a survey of the state of the art on schema information extraction approaches. We analyze and classify these approaches into three families: (1) approaches that exploit the implicit structure of the data, without assuming that some explicit statements on the schema are provided in the dataset; (2) approaches that use the explicit schema statements contained in the dataset to complement and enrich the schema, and (3) those that discover structural patterns contained in a dataset. We compare these studies in terms of their approach, advantages and limitations. Finally we discuss the problems that remain open.