Phrases and clauses are both components of sentences, but they serve different purposes. Phrases act as parts of speech, such as nouns, verbs, or adjectives, and they do not express a complete thought on their own. On the other hand, clauses are groups of words that contain a subject and a verb and can express a complete thought. They can function as complete sentences or as part of a larger sentence. Therefore, the given answer correctly explains that phrases simply act as parts of speech, while clauses are parts of whole thoughts.