Some examples of rhyme schemes are AABB, ABAB, ABCB, AAAB, ABBC and ABCA; each new sound at the end of a line of the poem is assigned a different letter. For example, if the first two lines of a poem end in cat and hat, these two lines are AA. If the next two lines end in son and done, these are BB and create a rhyme scheme of AABB.
A poem that reads "My cat is gray, my cat is fat, my cat is cute, I like my cat" has a rhyme scheme of ABCB. There are a plethora of rhyme schemes that can be created by giving each line of the poem a distinct sound.