Part of Speech

English words have been traditionally classified into eight lexical categories, or parts of speech (and are still done so in most dictionaries):
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 (or "exclamation")
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. This is because prototypical members of each class share the ability to change their form by accepting derivational or inflectional morphemes. The term form is used because it refers literally to the similarities in shape of the word in its pronunciation and spelling for each part of speech.[10]
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".

http://en.wikipedia.org/wiki/Part_of_speech