Classification is the assignment of entities to classes or groups. In community ecology, the customary input data are species abundances in a two-way samples-by-species data matrix. The resultant output from classification is either a nonhierarchical or hierarchical arrangement of samples, species, or both, as illustrated earlier in the introduction. An arranged data matrix may be viewed formally as merely a particular option for presenting the results of a hierarchical classification, but this option merits special consideration because it is quite useful and has had an important role from the beginnings of community classification. There are uncountable individual techniques for classification, but the central purpose of community classification is to summarize large community data sets.

Classification is a fundamental, ubiquitous mental activity (Goodall 1953; Sokal 1974; Blashfield & Aldenderfer 1978). Even simple animals must classify past sensory experiences as a prerequisite for future avoidance of deleterious actions and places and for seeking useful foods and habitats. The existence of language is predicated upon the classification of similar entities or concepts under words serving as class labels. It is not surprising that the first conceptual framework used to organize quantitative community data was classification. Becking (1957), Whittaker (1962), and Westhoff & Maarel (1978) trace the earliest origins of relevant concepts to about 1800 and the earliest substantial community classification to about 1900.

Early classification techniques were informal and subjective (Goodall 1953; Whittaker 1962; Sokal 1974). Several limitations were felt.