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.
The rhyme scheme of the poem "A Poison Tree" - written by Willian Blake, is as follows:
 
I was angry with my friend;A
I was angry with my friend;A
I was angry with my foe:B
I told it not, my wrath did grow.B
  
And I water' d it in fears,A
Night & morning with my tears: A
And I sunned it with smiles,B
And with soft deceitful wiles. B
  
And it grew both day and night. A
Till it bore an apple bright. A
And my foe beheld it shine,B
And he knew that it was mine. B
  
And into my garden stole, A
When the night had veiled the pole; A
In the morning glad I see; B
My foe outstretched beneath the tree.B
 
Important!
The overall rhyme scheme of the poem is \(AABB\).