Descriptive words generally appear in the form of adjectives, but adverbs, gerunds (verbs in the "ing" form) and even nouns can be descriptive words, depending on their usage in the sentence. An example for each type is, respectively, "tall," "quickly," "laughing" and "history."
Since any word that describes something can count as a descriptive word, there exist countless descriptive words in the English language. When people think about descriptive words, they generally think about adjectives, because adjectives, by definition, are words that define nouns. For example, in the sentence, "He is a tall man," "tall" describes the noun "man" and gives the reader a clearer picture of the person in question. All adjectives are indeed descriptive words, but not all descriptive words are necessarily adjectives. Adverbs are, for example, used to describe verbs and adjectives. In the sentence, "I walked quickly," "quickly" is a descriptive word, because it describes how I was walking. "I walked slowly" would, of course, paint a different picture.
Gerunds are also often used for description. This is shown in the example sentence, "I saw some laughing children on the playground." "Laughing," although a verb, is used here to describe the children. In this way, any verb can potentially be a descriptive word.
Even nouns can be descriptive words. For example, describing someone as a "history teacher" says much more about that person than simply describing him as a "teacher." Here, "history" is the descriptive word, describing the noun "teacher."