Definition of Alliteration:
The repetition of consonant sounds at the beginning of words.

Example of Alliteration #1:

Aeronautics, By Unknown

A flea and a fly in a flue,
Were imprisoned; now what could they do?
Said the fly, "let us flee."
"Let us fly," said the flea,
And they flew through a flaw in the flue.

This poem uses alliteration by repeating the consonant "F" in "Flea", "Fly" & "Flue".  Alliteration has an effect on rhythm & meter similar to that of internal rhyme.  

Example of Alliteration #2:

At the Wedding March, By Gerard Manley Hopkins

God with honour hang your head,
Groom, and grace you, bride, your bed
With lissome scions, sweet scions,
Out of hallowed bodies bred.

Alliteration plays a big role in this stanza.  Each line has an instance of consonant sounds being repeated for a desired effect, which is typical of alliteration.

alliteration poem
Alliteration in poetry.

Slideshow:    Alliteration

Alliteration Video

alliteration poem

See also:    Assonance    Consonance