Alliteration is the recurrence of initial consonant sounds. The repetition can be juxtaposed (and then it is usually limited to two words):

• Ah, what a delicious day!
• Yes, I have read that little bundle of pernicious prose, but I have no comment to make upon it.
• Done well, alliteration is a satisfying sensation.

This two-word alliteration calls attention to the phrase and fixes it in the reader's mind, and so is useful for emphasis as well as art. Often, though, several words not next to each other are alliterated in a sentence. Here the use is more artistic. And note in the second example how wonderfully alliteration combines with antithesis:

• I shall delight to hear the ocean roar, or see the stars twinkle, in the company of men to whom Nature does not spread her volumes or utter her voice in vain. --Samuel Johnson
• Do not let such evils overwhelm you as thousands have suffered, and thousands have surmounted; but turn your thoughts with vigor to some other plan of life, and keep always in your mind, that, with due submission to Providence, a man of genius has been seldom ruined but by himself. --Samuel Johnson
• I conceive therefore, as to the business of being profound, that it is with writers, as with wells; a person with good eyes may see to the bottom of the deepest, provided any water be there; and that often, when there is nothing in the world at the bottom, besides dryness and dirt, though it be but a yard and a half under ground, it shall pass, however, for wondrous deep, upon no wiser a reason than because it is wondrous dark. --Jonathan Swift