Information Extraction deals with the automatic extraction of information from unstructured sources. This field has opened up new avenues for querying, organizing, and analyzing data by drawing upon the clean semantics of structured databases and the abundance of unstructured data. The text surveys over two decades of information extraction research from various communities such as computational linguistics, machine learning, databases and information retrieval. Information Extraction provides a taxonomy of the field along various dimensions derived from the nature of the extraction task, the...