Adjectives are just one of several different types of noun modifiers, typically used to premodify or describe a noun. Do not confuse adjectives with nouns used attributively to modify other nouns. Adjectives have comparative and superlative degrees, can be used as predicate adjectives in copulae, and can themselves be modified by intensifiers and adverbs but not by other adjectives. Nouns in attribution fail all those tests.
Examples of adjectives are: beautiful, nice, colorful.
Examples of non-adjectives are: a, the, these, thirteen, many, dog, hazelnut.
Adjectives are usually used either predicatively or else prenominally, but some postpositive examples are possible. Unlike nouns, adjectives enjoy comparative and superlative degrees and cannot be modified by other adjectives.