A diphthong is a combination of 2 vowel sounds. it is a word used to describe pronunciation and does not consider the spelling of a word. For example: cow, drought, noun, and vowel all contain the diphthong 'au'. A vowel team is a combination of 2 vowels that make a certain sound in a word. This term is concerned with spelling. For example: piece, need, plead, and seize all have 2 owels working together to make the long e sound.