Sweet basil is light green with wide leaves while Thai basil has purple stems and flowers and spear-like leaves
Thai basil, or Asian basil (húng quế in Vietnamese) is a type of sweet basil native to Southeast Asia that has been cultivated to provide distinctive traits. Its flavor is more stable under high or extended cooking temperatures than that of sweet basil. Thai basil exhibits small, narrow leaves and purple stems, with a mauve (pink-purple) flower. One cultivar commonly grown in the United States is 'Queen of Siam'.