is when a poem follows a distinct pattern of sound at the end of each line.  This provides a momentum for the poem, keeping the reader engaged for what comes next.  When identifying, label each matching sound with a corresponding letter of the alphabet, starting with A.  Look for it here:

"Sick" by Shel Silverstein

Choose by S.M.(M).L.

Strawberries are rubescent
as if glowing from within.
As for berries, they have the sweetest scent;
the taste is best when dribbled down your chin.
Like a heart, they are tender.
When plucked too hard, they will bruise,
but like the broken heart, they too will stain
the hands of the offender.
So when you pick, make sure with care you choose.