Answer:
A rhyme scheme is the pattern of rhymes at the end of each line of a poem or song. It is usually referred to by using letters to indicate which lines rhyme; lines designated with the same letter all rhyme with each other.
Let's look at an example I found on G o o g l e
From all the children in my school,
I may not be the tallest,
From all the voices in the world
Mine may be the smallest.
Here, the rhyme scheme is ABCB. Let me show you why.
First, lets mark the first sentence as A
From all the children in my school, A
I may not be the tallest,
From all the voices in the world
Mine may be the smallest.
Next, lets mark the second sentence as B
From all the children in my school, A
I may not be the tallest, B
From all the voices in the world
Mine may be the smallest.
Now, the third one, mark as C
From all the children in my school, A
I may not be the tallest, B
From all the voices in the world C
Mine may be the smallest.
Now, instead of marking the last one D, lets mark it B again!
From all the children in my school, A
I may not be the tallest, B
From all the voices in the world C
Mine may be the smallest. B
I guess now your question is "Why didn't you mark the last sentence as D, and why did you mark it B?
Well, Lets look at the poem again!
From all the children in my school, A
I may not be the tallest, B
From all the voices in the world C
Mine may be the smallest. B
Now, do the last letters of line A and B, rhyme? No, right! That's why we marked them with different letters! But, as you can see, the words at the end of the sentences, tallest and smallest rhyme! So, instead of marking that as D, we marked it as B again, because they rhymed! It looks very tough, but it's very simple! Hope this helped!