In linguistics, a closed class (or closed word class) is a word class to which no new items can normally be added, and that usually contains a relatively small number of items. Typical closed classes found in many languages are adpositions (prepositions and postpositions), determiners, conjunctions, and pronouns.
Contrastingly, an open class offers possibilities for expansion. Typical open classes such as nouns and verbs can and do get new words often, through the usual means such as compounding, derivation, coining, borrowing, etc. 
A closed class may get new items through these same processes, but the change takes much more time. The closed class is normally viewed as part of the core language and is not expected to change. Most readers can undoubtedly think of new nouns or verbs entering their lexicon, but it's very unlikely that they can recall any new prepositions or pronouns appearing in the same fashion.
Different languages have different word classes as open class and closed class – for example, in English, pronouns are closed class and verbs are open class (see for example the contentious topic of gender-neutral pronouns in English and how common verbing is), while in Japanese, pronouns are open class, while verbs are closed class – to form a new verb, one suffixes 〜する (-suru, "to do") to a noun – for example, "to exercise" is 運動する – "to do exercise".
- http://strazny.com/encyclopedia/sample-function-words.html Closed class words
- http://www.ucl.ac.uk/internet-grammar/wordclas/wordclas.htm Both open and closed class words
- Dixon, R. M. W. (1977). Where have all the adjectives gone?. Studies in language, 1, 19–80.