On the basis of their grammatical behavior the words of a language are divided into several classes. These different classes of words are called the parts of speech. Languages differ in the parts of speech they have. English, for example has eight parts of speech: noun, pronoun, adjective, adverb, preposition, conjunction, verb and interjection.
Some dictionaries recognize more than eight parts of speech. For example, determiners and degree modifiers are also sometimes considered as different parts of speech.
Words are assigned to parts of speech according to their grammatical behavior. For example, words are classified according to the positions in which they can occur in a sentence and the way they change their forms for grammatical reasons.
Words placed together in a single part of speech have important grammatical properties in common, but that doesn’t mean that all the words in a single part of speech have grammatical properties which are entirely identical.
In English, it is possible to assign a single word to two or more parts of speech. For example, book is a noun in the sentence ‘Give me that book’ but a verb in the sentence ‘Book your tickets early’. Similarly fast is an adjective in the sequence ‘a fast car’ but an adverb in the sentence ‘He drove fast’.
A few words exhibit behavior that cannot be assigned to any part of speech at all. English examples include the negative ‘not’ and the polite ‘please’.