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We have three articles in the English language: a, an, and the. The word the is the definite article. The words a and an are indefinite articles.

Grammarians often go on for pages analyzing the differences between a-an and the. Generally, you use the word the when you're referring to a known entity or a single example. You use the word a or an when you're referring to something in general, not a specific one or ones.

You'll find a complete discussion in the section on Adjectives in Parts of Speech on Grammar.com. Click here for the beginning of that discussion.