PART OF SPEECH

In grammar, a part of speech (also a word class, a lexical class, or a lexical category) is a linguistic category of words (or more precisely lexical items), which is generally defined by the syntactic or morphological behaviour of the lexical item in question.
Common linguistic categories include noun and verb, among others.
There are open word classes, which constantly acquire new members.
There are closed word classes, which donot acquire new members frequently.
Almost all languages have the lexical categories noun and verb, but beyond these there are significant variations in different languages.
For example, Japanese has as many as three classes of adjectives where English has one; Chinese, Korean and Japanese have nominal classifiers whereas European languages do not; many languages do not have a distinction between adjectives and adverbs, adjectives and verbs or adjectives and nouns, etc. This variation in the number of categories and their identifying properties entails that analysis be done for each individual language. Nevertheless the labels for each category are assigned on the basis of universal criteria.
By the end of the 2nd century BCE, the classification scheme had been expanded into eight categories, seen in the Art of Grammar :

Noun: a part of speech inflected for case, signifying a concrete or abstract entity
Verb: a part of speech without case inflection, but inflected for tense, person and number, signifying an activity or process performed or undergone
Participle: a part of speech sharing the features of the verb and the noun
Interjection: a part of speech expressing emotion alone
Pronoun: a part of speech substitutable for a noun and marked for a person
Preposition: a part of speech placed before other words in composition and in syntax
Adverb: a part of speech without inflection, in modification of or in addition to a verb, adjective, clause, sentence, or other adverb
Conjunction: a part of speech binding together the discourse and filling gaps in its interpretation

The Latin grammarian Priscian (fl. 500 CE) modified the above eightfold system, substituting "interjection" for "article".
It was not until 1767 that the adjective was taken as a separate class.
Traditional English grammar is patterned after the European tradition above, and is still taught in schools and used in dictionaries.
It names eight parts of speech: noun, verb, adjective, adverb, pronoun, preposition, conjunction, and interjection.

Controversies

Since the Greek grammar of 2nd century BCE, parts of speech have been defined by morphological, syntactic and semantic criteria.
However, there is currently no generally agreed-upon classification scheme that can apply to all languages, or even a set of criteria upon which such a scheme should be based.

English

English words have been traditionally classified into eight lexical categories, or parts of speech:

Noun: any abstract or concrete entity; a person (police officer, Michael), place (coastline, London), thing (necktie, television), idea (happiness), or quality (bravery)
Pronoun: any substitute for a noun or noun phrase
Adjective: any qualifier of a noun
Verb: any action (walk), occurrence (happen), or state of being (be)
Adverb: any qualifier of an adjective, verb, clause, sentence, or other adverb
Preposition: any establisher of relation and syntactic context
Conjunction: any syntactic connector
Interjection: any emotional greeting

Although these are the traditional eight English parts of speech, modern linguists have been able to classify English words into even more specific categories and sub-categories based on function.

The four main parts of speech in English, namely nouns, verbs, adjectives and adverbs, are labelled form classes as well.
Neither written nor spoken English generally marks words as belonging to one part of speech or another, as they tend to be understood in the context of the sentence.
Words like neigh, break, outlaw, laser, microwave and telephone might all be either verb forms or nouns.

Although -ly is a frequent adverb marker,

not all adverbs end in -ly (-wise is another common adverb marker) and

not all words ending in -ly are adverbs.

For instance, tomorrow, fast, very can all be adverbs,

while early, friendly, ugly are all adjectives (though early can also function as an adverb).

Verbs can also be used as adjectives (e.g. "The astonished child watched the spectacle unfold" instead of the verb usage "The unfolding spectacle astonished the child"). In such cases, the verb is in its participle form.
In certain circumstances, even words with primarily grammatical functions can be used as verbs or nouns, as in "We must look to the how's and not just the why's" or "Miranda was to-ing and fro-ing and not paying attention".

Functional classification

The study of linguistics has expanded the understanding of lexical categories in various languages and allowed for better classifying words by function. Common lexical categories in English by function may include: