Iambic Pentameter, Ottava Rima
Byron chose a very specific poetic structure known as Ottava Rima to write Don Juan. This structure involves a rhyme scheme of ABABABCC (where each letter stands for a line's end rhyme). It's also written—for the most part—in a rhythmic pattern called iambic pentameter. Put simply, iambic pentameter involves lines of five ("penta-" means five) iambs, which are two-syllable pairs in which the first syllable is unstressed and the second stressed (making a daDUM beat). This pattern holds throughout the poem, with the exception of a syllable here or there. (Hey, you try writing five thousands lines without breaking the meter.) Check out Canto I, stanza 9 to see this pattern in action:
His father's name was Jose—Don, of course,—
A true Hidalgo, free from every stain
Of Moor or Hebrew blood, he traced his source
Through the most Gothic gentlemen of Spain;
A better cavalier ne'er mounted horse,
Or, being mounted, e'er got down again,
Than Jose, who begot our hero, who
Begot—but that's to come—Well, to renew
With the exception of the hiccup that starts the fourth line in this stanza, the pattern of iambs is one you can set your watch to: daDUM, daDUM, daDUM, daDUM, daDUM.
Of course, the question remains: why did Byron choose this form? For an answer, we turn to the last two lines of each stanza, the final couplet. The purpose of the couplet rhyme at the end of each stanza is designed to give a humorous conclusion to the six lines that come before it. Take for example the lines, "A dull and family likeness through all ages,/ Of no greater promise for poetic pages" (14.15). These lines basically sum up an earlier description of upper-class people by saying they're too boring for the pages of epic poetry. It's the ABABAB build-up that tends to always culminate in Byron's satirical final couplet, which is calculated to hit his readers like a punch in the gut. Since there are more than two thousand stanzas in this poem, that means more than two thousand punches to the gut—umf.