A given occurrence of an adjective can generally be classified into one of four kinds of uses:
Attributive adjectives are part of the noun phrase headed by the noun they modify; for example, happy is an attributive adjective in "happy people". In some languages, attributive adjectives precede their nouns; in others, they follow their nouns; and in yet others, it depends on the adjective, or on the exact relationship of the adjective to the noun. In English, attributive adjectives usually precede their nouns in simple phrases, but often follow their nouns when the adjective is modified or qualified by a phrase acting as an adverb. For example: "I saw three happy kids", and "I saw three kids happy enough to jump up and down with glee." See also Postpositive adjective.
Predicative adjectives are linked via a copula or other linking mechanism to the noun or pronoun they modify; for example, happy is a predicate adjective in "they are happy" and in "that made me happy." (See also: Predicative expression, Subject complement.)
Absolute adjectives have meanings that are implicitly unable to be used comparatively or superlatively; for example, dead is an absolute adjective, as the words deader or deadest are not normally used to describe states of death.
Nominal adjectives act almost as nouns. One way this can happen is if a noun is elided and an attributive adjective is left behind. In the sentence, "I read two books to them; he preferred the sad book, but she preferred the happy", happy is a nominal adjective, short for "happy one" or "happy book". Another way this can happen is in phrases like "out with the old, in with the new", where "the old" means, "that which is old" or "all that is old", and similarly with "the new". In such cases, the adjective functions either as a mass noun (as in the preceding example) or as a plural count noun, as in "The meek shall inherit the Earth", where "the meek" means "those who are meek" or "all who are meek".
In many languages, including English, it is possible for nouns to modify other nouns. Unlike adjectives, nouns acting as modifiers (called attributive nouns or noun adjuncts) usually are not predicative; a beautiful park is beautiful, but a car park is not "car". In plain English, the modifier often indicates origin ("Virginia reel"), purpose ("work clothes"), or semantic patient ("man eater"), however, it may generally indicate almost any semantic relationship.