is the repetition of identical or similar consonant sounds, normally at the beginning of words. Coming up with alliterative phrases (rubber baby buggy bumpers, anyone?) can be a fun game, but it can also be a great literary device, and many authors have used it to subtly enhance the meaning of their words.
For example, the third stanza of Edgar Allan Poe's The Raven begins with the repeated use of an s sound (a special type of alliteration called sibilance), which mimics the "uncertain rustling" that he's writing about. In the second line, he continues the alliteration, only this time with the f sound.
And the silken, sad, uncertain rustling of each purple curtain
Thrilled me — filled me with fantastic terrors never felt before;
Alliteration deals with consonant sounds. The vowel-sound equivalent is assonance, the repetition of vowel sounds. The last stanza of Poe's The Raven begins thus:
And the Raven, never flitting, still is sitting, still is sitting
Notice how the short i sound arrives with the word flitting and just stays there until the end of the line (along with the s and t sounds — more alliteration), just like the Raven that Poe describes.
Not only can well-placed alliteration and assonance add a second layer to the meaning of text, but both also bring the briefest bliss to the reader of these devices. In short, they're just fun to read!