"Red," "tall," "large," "small" and "long" are all examples of adjectives. Adjectives are defined as words that function to modify or change nouns and pronouns.
Nouns are people, places or things. In order to modify nouns, adjectives are used. An example of a sentence that uses an adjective is: "The blue shirt arrived Thursday." "Shirt" is the noun because it is the thing, while "blue" changes the shirt completely. It could have been black or purple, but in this sentence, it is blue. The adjective "blue" modifies the noun "shirt." In the sentence, "She was happy," "she" is the pronoun, a word that replaces a noun, and "happy" is the adjective because it changes what "she" is.