Every word belongs to a word class which summarises the ways in which it can be used in grammar. The major word classes for English are: noun, verb, adjective, adverb, preposition, determiner, pronoun, conjunction. Word classes are sometimes called parts of speech.
A word class is a group of words which show similar grammatical behaviour. For example, words which belong to the class of nouns occur as the Heads of noun phrases, can be preceded by determiners, and so on. The National Curriculum prefers the label 'word types'.