The way in which a poem rhymes. There are many rhyme schemes, some simple, some complex. Rhyme schemes are usually illustrated by a lowercase letter of the alphabet denoting each line. Take for example in this poem entitled "Song: To Celia" by Ben Jonson:

Drink to me only with thine eyes,
And I will pledge with mine;
Or leave a kiss but in the cup,
And I'll not look for wine.
The thirst that from the soul doth rise,
But might I of Jove's nectar sup,
I would not change for thine.

I sent thee late a rosy wreath,
Not so much honoring thee,
As giving it a hope, that there
It could not withered be.
But thou thereon did'st only breathe,
And sent'st it back to me;
Since when it grows and smells, I swear,
Not of itself, but thee.

The rhyme scheme would be shown like this:

a
b
c
b
a
b
c
b

d
e
f
e
d
e
f
e

or this: a b c b a b c b d e f e d e f e

Of the different types of poems, the type of poem that has the most rigourously enforced rhyme scheme is the sonnet. Ballads, on the other hand, often have rhyme schemes, but due to convention, not a strict arrangement.