Word classes are categories of word. The categories are defined by what the word does. Some word classes are open, which means that new words can be added to them, and others are closed, which means no new words can be added. Nouns and verbs are open word classes and prepositions and determiners are closed.

Example
Nouns, verbs and prepositions are word classes.

In the classroom
Adding new words to open word classes is enjoyable and motivating for learners. This can be done as mini-presentations, with learners taking turns to present a word they have found, or as a class project, or in the form of regular quizzes.