Both alliteration and assonance are useful literary devices not only for analyzing and interpreting poetry, but for including in your own.
Alliteration occurs when words begin with the same sound. This serves as a way to link a poem together, adding to its cohesiveness as a whole. Edgar Allan Poe's "The Raven" gives an excellent example. Poe uses alliteration from the very beginning, describing the speaker "weak and weary" looking over "quaint and curious" books. As the action picks up, so does the alliteration ("While I nodded, nearly napping, suddenly there came a tapping"), drawing the words and sounds together while drawing the reader into the story of the poem.
Alliteration not only brings the whole of the poem together, but often pulls words and phrases together, emphasizing them. In "The Raven," the narrator is "weak and weary," while his love Lenore is "rare and radiant" and the raven itself "ghastly grim". The time of year, December, is "distinctly" remembered and closely connected with "dying". When darkness is introduced, the intensity increases, aided by alliteration in the lines:
Deep into that darkness peering, long I stood there wondering, fearing,
Doubting, dreaming dreams no mortal ever dared to dream before;
The hard sound of d repeating acts like a drum, or like the knocking that wakes the narrator from his reveries. It rings ominous, a tone that continues throughout the poem. The repetition of the word "tapping" also invokes the actual sound. Though "tap" may be more accurately classified as onomatopoeia, its repetition here qualifies it as alliteration also, enhancing the pull which draws the reader into the world of the poem.
Assonance, closely related to alliteration, is the repetition of sounds, particularly vowels, in the middle of words. This can also be used to draw parts and ideas of a poem together. Alfred, Lord Tennyson uses assonance effectively in his famed poem "The Lady of Shalott."
On either side of the river lie
Long fields of barley and of rye,
That clothe the wold and meet the sky;
And through the field the road runs by
To many-towered Camelot;
Do not confuse assonance with rhyme, however. Rhyme is reserved for the ends of the words. In the above stanza, it's true that lie, rye, sky and by all rhyme. Yet they are also linked to "either" and "side" by the long "i" sound, which is assonance. Basically, some assonance is rhyme, but not all rhyme is assonance.
In "The Lady of Shalott," Tennyson uses assonance throughout the first line, then continues the chain of sound with rhyme, uniting the stanza as a whole. The last word, "Camelot," breaks the flow, and therefore stands out from the rest of the words. This signifies importance. The reader stops, must ponder the word "Camelot" and all that it implies, much in the way that a "many-towered" castle must have caused travelers to pause and notice it among rolling fields and lowlier buildings.
Other instances of assonance serve to pair descriptions to nouns. The Lady lives on a "silent isle", where she looks down upon the world, on the "reaper weary," "surly village-churls", "damsels glad", "knights [that] come riding", etc. Though descriptions can certainly be effective without assonance, it helps with the cohesiveness of the poem. It is particularly pleasing when read aloud.
Thus we see that assonance and alliteration not only serve to emphasize words, lines and stanzas that go together, but also those that stand apart. By noting where a poet has used these devices, the reader can discern themes and meanings within the poem. Likewise, poets who wish to emphasize certain aspects or descriptions can successfully employ these terms.