Alliteration is a device used in writing, often in poetry, in which the beginning consonant is repeated in all or most of the words in a line or sentence. "She sells seashells by the seashore" is one common example. "Peter Piper picked a peck of pickled peppers" is another. The term "alliteration" is sometimes used interchangeably with the term "consonance," which means using the same consonant anywhere within a word, not just at the beginning, frequently in a line or sentence.
Alliteration is used to make a sentence or line of poetry more pleasing or interesting to the ear. It is a device frequently used in slogans, catchphrases and article titles. Alliteration also is a common component of verbal expressions such as "fit as a fiddle" and "trick or treat."
The use of alliteration in poetry and literature dates back to the very earliest written works. It was widely used in the story "Beowulf," for instance. Alliteration was very popular in the oldest known Germanic and Norse works, including the Edda, and may have originated in Old English or a related group of Germanic languages.
Alliteration can make prose or poetry interesting or more memorable, especially when read aloud. The device tends to make catchphrases and slogans more memorable. Alliteration is often used in children's literature because it adds auditory interest to the tale when it's read aloud.
It's easy to overuse alliteration. The device is widely used by some copy editors who write newspaper headlines because it can make a title catchy. However, it can also come across as "cutesy" and wouldn't necessarily be appropriate for serious or sad news. While a smattering of alliteration in a poem can make a line pop, extensive use of this literary device is currently out of favor at most journals and magazines that publish modern poetry. Small amounts of alliteration used subtly make a line pleasing to the ear, but excessive use of alliteration can give a poem a singsong or childish tone.