Alliteration is the repetition of consonant sounds (not vowels - AEIOU) in a string of two or more words. For example: grumpy girls get grouchy See how the 'g' repeats its sound in each word?
Artistic apes arranged apples.
Betty blanched and baked before barbecuing.
Crazy cats kept coming constantly.
Dormant dogs don't dust davenports.
Elegant elephants eat eggs.
Four fragrant farmers fanned flames.
Ghostly grimaces and growls grew ghastly.
However helpful, Harry hears hooligans hum.
Jumping jacks jangle Jill's junipers.
Kitchen cafe curtains cling closely.
Large llamas lounge lazily.
Mad men munch mouthfuls of mango.
Nosy nurses never know enough.
Obnoxious otters offer oats.
Pretty purple pickles pout perfectly.
(See illustration above.)
Quiet Quakers quit quilting.
Rested ranchers roasted rare rump roasts.
She shelves sushi with a short shovel.
Tart retorts retain temper tantrums.
Very vivacious velvet-clad vixens voraciously ate Velveeta.
Wild warriors were weary with waywardness.
eXpect eXcitement eXactly at the eXit.
She used your yellow yardstick yearly.
Zelda raised a zillion for the zoo.
If you do a Google search, you will find a lot of sites that have "alliteration poems." Truthfully, there is no such thing. there are, however, poems that employ this device. It is part of the delicious family of word play.
Just a note: both assonance and consonance are closely related to this topic. Assonance is the repetition of vowel sounds. (Example: "bee" and "keep") Consonance is the repetition of consonant sounds. (Example: "borders" and "birders") In "consonance." The only part of the word they DON'T have in common is the vowels.
Also, if you pay attention to all the examples above, you will notice that some of the repetitive sound comes from the middle of the word --- not just from the first letter. Another example: birds in the abbey. The 'b' in birds match the 'b' in abbey.
The human ear likes rhyme, rhythm and repetition. That is what makes for good speech, good writing and good poetry, (whether it rhymes or not). Once you make the alphabet your plaything, the possibilities are endless.