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 you'll be introduced to the eight traditional parts of speech.
Learning the names of the parts of speech probably won't make you witty, wealthy, or wise. In fact, learning just the names of the parts of speech won't even make you a better writer. But you will gain a basic understanding of the English language, which should help you follow the other lessons here at About.com Grammar and Composition. And those lessons will help you to improve your writing.
As you study the table at the bottom of the page, keep in mind that only interjections ("Yes!") have a habit of standing alone (or alongside complete sentences). The other parts of speech--nouns, pronouns, verbs, adjectives adverbs, prepositions, and conjunctions--come in many varieties and may appear just about anywhere in a sentence. To know for sure what part of speech a word is, we have to look not only at the word itself but also at its meaning, position, and use in a sentence.