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:
Let us take an example to learn about a different pattern - "The Road not Taken", written by Robert Frost.
Example:
Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveller, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
  
The words wood, stood and could are rhyming in this stanza. So, it can be named as:
Two roads diverged in a yellow wood,
A
And sorry I could not travel bothB
And be one traveller, long I stoodA
And looked down one as far as I couldA
To where it bent in the undergrowth;
B
It can be seen that this poem follows the ABAAB pattern.
Furthermore, if we read the full poem, we see that the pattern continues in this form:
Two roads diverged in a yellow wood,A
And sorry I could not travel bothB
And be one traveller, long I stoodA
And looked down one as far as I couldA
 To where it bent in the undergrowth;
B
  
Then took the other, just as fair,A
And having perhaps the better claim,B
Because it was grassy and wanted wear;A
Though as for that the passing thereA
 Had worn them really about the same.B
  
And both that morning equally layA
In leaves no step had trodden black.B
Oh, I kept the first for another day!A
Yet knowing how way leads on to way,A
 I doubted if I should ever come back.B
  
I shall be telling this with a sighA
Somewhere ages and ages hence;B
Two roads diverged in a wood, and IA
I took the one less travelled by,A
And that has made all the difference.
B
The overall rhyme scheme in this poem is ABAAB.