Diphthongs are complex vowel sounds that begin with one vowel sound and glide into another within the same syllable, while monophthongs are simple, single vowel sounds that have a steady quality without any change in articulation. Understanding these distinctions is crucial for pronunciation and phonetics, as diphthongs can significantly affect the meaning of words in languages, including Latin. The ability to recognize and pronounce these vowel types accurately is essential for mastering Latin pronunciation and understanding its phonological system.