Humans are explorers by nature. Almost all searches are exploratory to a certain extent. As a result of the subdivision of the information seeking domain, exploratory search has become a new research focus arousing extensive attention. This chapter introduces the concept of exploratory search and illustrates its basic theoretical foundations, clarifying its complex meaning from the aspects of the problem context and the search process. Four different methods of classifying search results are identified based on a survey of existing exploratory search systems, including hierarchical classification, faceted classification, dynamic clustering, and social classification. Their inherent characteristics and practical applications are reviewed in detail, and the visualization support for presenting the classified search results is explored in addition. The development trends of the exploratory search field are predicted according to the social nature of information seeking.