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 the AABB pattern.
Example:

2. Let us take another example to learn about a different pattern - "Stopping by Woods on a Snowy Evening", written by Robert Frost.

Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.

The words know, though and snow are rhyming in this stanza. So, it can be named as:

Whose woods these are I think I know.
A
His house is in the village though;A
He will not see me stopping hereB
To watch his woods fill up with snow.A
It can be seen that this poem follows the AABA pattern.
Furthermore, if we read the full poem, we see that the pattern continues in this form:
Whose woods these are I think I know.A
His house is in the village thoughA
He will not see me stopping hereB
To watch his woods fill up with snow.A
  
My little horse must think it queerB
To stop without a farmhouse nearB
Between the woods and frozen lakeC
The darkest evening of the yearB
  
He gives his harness bells a shakeC
To ask if there is some mistake.C
The only other sound’s the sweepD
Of easy wind and downy flake.C
  
The woods are lovely, dark and deep,D
But I have promises to keep,D
And miles to go before I sleep,D
And miles to go before I sleep.D
The overall rhyme scheme in this poem is AABA BBCB CCDC DDDD.