This is a very simplistic explanation of adjectives. There are many types, including absolute, attributive, comparative, superlative, compound, coordinate, copulative, and predicate. Then there are adjective clauses and adjective phrases, along with prepositional and participial. Never fear! We'll keep this article easy and painless.
ADJECTIVE: a word or phrase that describes what kind, how many, or which one. Adjectives can consist of a single word, a phrase, or a clause. Adjectives modify (or explain) the meaning of nouns (see below) and pronouns (see below).
NOUN: name of a person, place, thing, activity, idea, quality.
PRONOUN: a word used in place of a noun, e.g., I, you, he, she, it, we, they, who, whose, which, what, that, these, those, each, either, any, anyone, someone, myself, yourself, himself, etc.
You usually find adjectives before the nouns they modify (her gray hair) but they can come at the end of a sentence (Her hair is gray).
Some words are also used as both adjectives and adverbs, e.g., "best, deep, fast, hard, quick and long." You may need to look at the sentence and see the function in order to decide.
ADJECTIVE: He's a *fast runner
ADVERB: He runs *fast
Adjectives can only modify nouns or pronouns. And if I may be blunt, don't use adjectives (or adverbs) if you don't need them. They have their place, but sometimes dilute the power of a sentence.