Part of speech is the common name for a word class -- a category into which words are placed according to the work they do in a sentence. Here are some examples:
1. Noun: any abstract or concrete entity; a person, place, thing, idea, or quality
2. Pronoun: any substitute for a noun or a noun phrase
3. Adjective: any qualifier of a noun or a pronoun
4. Verb: any action, occurrence, or state of being
5. Adverb: any qualifier of an adjective, verb, clause, sentence, or other adverb
6. Preposition: any establisher of relation and syntactic context
7. Conjunction: any syntactic connector
8. Interjection: any short utterance that usually expresses emotion
9. Article: any word that is used to introduce a noun
These categories are called POS, and are useful for resolving ambiguity and many other language processing tasks.
Part-of-speech tagging (POS-tagging)
The process of categorizing words into their parts of speech and labeling them accordingly is known as POS-tagging. Typically, POS-tagging will identify sub-class attributes (e.g. singular vs. plural nouns, present vs. past tense verbs, etc.). This step is useful for subsequent syntactic parsing and word sense disambiguation.