An adjective is a word used to modify, or describe, a noun or pronoun. An adjective usually answers one of these questions: Which one? What kind? How many?
Adjectives usually precede the words they modify. They may also follow linking verbs, in which case they describe the subject. (See 41b.)
The definite article the and the indefinite articles a and an are also classified as adjectives.
Some possessive, demonstrative, and indefinite pronouns can function as adjectives: their, its, this, all (see 40b). And nouns can function as adjectives when they modify other nouns: apple pie (the noun apple modifies the noun pie; see 40a).
TIP: You can find more details about using adjectives in 27.