This article presents and discusses definitions of the term ‘classification’ and the related concepts ‘concept’, ‘categorization’, ‘ordering’, and ‘taxonomy’. It further presents and discusses theories of classification including the influences of Aristotle and Wittgenstein. It presents different views on forming classes, including logical division, numerical taxonomy, historical classification, hermeneutical and pragmatic/critical views. Finally, issues related to artificial versus natural classification and taxonomic monism versus taxonomic pluralism are briefly presented and discussed.