Open classes of words are the classes that may continually be added to. For instance, Verbs are an Open class of words because new verbs may be created at any time and, in fact, are created all the time. "Texting" is a new verb formed from the noun "text" as in the new compound noun "text message." Which points out that Nouns are also an Open class of words. Other new nouns or new meanings of old nouns are widget (1925-1930), mouse (computer related useage), upquark, and stranglet. Adjectives and Adverbs are other Open classes of words as new adjectives and adverbs can be and are created, though less readily than nouns and verbs.
Closed classes of words are those that do not change in terms of the collection of words therein. Determiners are a Closed class of words: any, some, a, each, etc. won't be added to. Prepositions and Conjunctions are the two other word classes that are Closed and that will not be added to. One subgroup of words is also Closed. The Pronoun subgroup of the Noun class, which is Open, is another Closed class: we will not have any new pronouns...unless discoveries of extraterrestial life forms demands it.