An adjective is a word that describes a noun. There are two kinds: attributive and predicative.

An adjective is used attributively when it stands next to a noun and describes it:
The black cat climbed a tree.

NOTE: The verb participle forms can be used as adjectives:
The man felt a paralyzing fear. Flavored oatmeal tastes better than plain oatmeal.

The usual place of the adjective in English is in front of the noun. You can have a whole string of adjectives if you like: The tall thin evil-looking cowboy roped the short, fat, inoffensive calf.

Sometimes, for rhetorical or poetic effect, the adjective can come after the noun:
Sarah Plain and Tall (book title)
This is the forest primeval.

An adjective is used predicatively when a verb separates it from the noun or pronoun it describes:
The umpire was wrong.
The crowd was furious.
She seems tired today.
This soup tastes bad.
The dog’s coat feels smooth.

The verbs that can be completed by predicate adjectives are called being verbs or copulative verbs. They include all the forms of to be and sensing verbs like seem, feel, and taste.

Adjective Classifications
qualitative: good, bad, happy, blue, French
possessive: my, thy, his, her, its, our, your, their
relative and interrogative: which, what, whatever, etc.
numeral: one, two, second, single, etc.
indefinite: some, any, much, few, every, etc.
demonstrative: this, that, the, a (an), such

NOTE: the demonstrative adjectives the and a (an) are so important in English that they have a special name: articles. They are discussed separately (i.e. coming soon).