What is it called when each word of the sentence starts with the same letter?

Alliteration. From Merriam-Webster Online: al·lit·er·a·tion ; Pronunciation: : \ə-ˌli-tə-ˈrā-shən\ ; Function: : noun ; Etymology: : ad- + Latin littera letter ; Date: : circa 1624 : the repetition of usually initial consonant sounds in two or more neighboring Words or syllables (as wild and woolly, threatening throngs) -called also head rhyme initial rhyme