In natural language understanding, there is a hierarchy of lenses through which we can extract meaning - from words to sentences to paragraphs to documents. At the document level, one of the most useful ways to understand text is by analyzing its topics.
J. Eisenstein, B. O'Connor, N. Smith, and E. Xing. Proceedings of the 2010 Conference on Empirical Methods in Natural Language Processing, page 1277--1287. Stroudsburg, PA, USA, Association for Computational Linguistics, (2010)