Languages organize their parts of speech into classes according to their functions and positions relative to other parts. All languages, for instance, make a basic distinction between a group of words that prototypically denote things and concepts and a group of words that prototypically denote actions and events. The first group, which includes English words such as "dog" and "song," are usually called nouns. The second, which includes "run" and "sing," are called verbs. Other common categories are adjectives, words that describe properties or qualities of nouns such as "red" or "big".

The word classes also carry out differing functions in grammar. Prototypically verbs are used to construct predicates, while nouns are used as arguments of predicates. In a sentence such as "Sally runs," the predicate is "runs," because it is the word that predicates a specific state about its argument "Sally." Some verbs such as "curse" can take two arguments, e.g. "Sally cursed John." A predicate that can only take a single argument is called intransitive, while a predicate that can take two arguments is called transitive.

Many other word classes exist in different languages, such as conjunctions that serve to join two sentences and articles that introduces a noun.