Theory:

The pattern of rhymes at the ending of each line in a poem is called a rhyme scheme. Letters (A,B,C...) are usually used to express which lines rhyme. Verses that are designated with the same letter are said to rhyme with each other. It is also known as an arrangement of rhymes in a stanza or a poem.
Example:
1. For easier understanding of the concept, let us take a famous nursery rhyme, written by Jane Taylor as an example.
 
Twinkle twinkle little star,
How I wonder what you are.
Up above the world so high
Like a diamond in the sky.
 
We see that the first two lines rhyme with each other (star-are); the second two lines rhyme with each other (high-sky). Let us name each line as A, B depending on the words that rhyme with each other.
 
Twinkle twinkle little star,
A
How I wonder what you are. A
Up above the world so high,B
Like a diamond in the sky.
B
It can thus be seen that, this poem follows AABB pattern.
Rhyme scheme of the poem "On the Grasshopper and Cricket":
The poetry of earth is never dead
A
When all the birds are faint with the hot sunB
And hide in cooling trees, a voice will runB
From hedge to hedge about the new-mown meadA
That is the grasshopper’s — he takes the leadA
In summer luxury — he has never doneB
With his delights, for when tired out with funB
He rests at ease beneath some pleasant weedA
 
The poetry of earth is ceasing neverC
On a lone winter evening when the frostD
Has wrought a silence, from the stone there shrillsE
The cricket’s song, in warmth increasing everC
And seems to one in drowsiness half lostD
The grasshopper’s among some grassy hillsE
It can thus be seen that, this poem also follows ABBA ABBA CDE CDE pattern.