This book deals with the traditional problem of the classification of linguistic units, with a primary focus on word classes. The approach is descriptive rather than theoretical, and is based on the use of distinctive features analogous to the ones used in phonology, which entails a radical reworking of the traditional classification. The first part presents some basic notions such as the use of distinctive features and the role of word classes in grammar; classification by prototypes; and the use of world knowledge as a resource to assign thematic relations to constituents in the sentence. In the second part, some descriptive problems are examined, namely the classification of verbs according to valency; connectives, adverbs, and the internal constituents of the NP; and the classification of units larger than words. This book will be of use as a guide for linguists working on the description of natural languages, as well as a resource for students on courses in linguistic theory and description.