A rhyme scheme is the pattern of rhyming lines in a poem. It is usually referred to by using letters to indicate which lines rhyme. For example ABAB indicates a four-line stanza in which the first and third lines rhyme, as do the second and fourth. Here is an example of this rhyme scheme from To Anthea, Who May Command Him Any Thing by Robert Herrick:
Bid me to weep, and I will weep,
While I have eyes to see;
And having none, yet I will keep
A heart to weep for thee.
Sounds in red are marked with the letter A
Sounds in blue are marked with the letter B
Sounds that almost rhyme are called "slant rhymes" or "near rhymes" (for instance, "fat" and "cant").
Rhyme is determined by sound, not spelling, so don’t get fooled. Which of these two pair of words rhyme?
puff / enough ← Even though these are spelled differently, these words sound similar
through / though ← Even though these are spelled similarly, these words sound different
The Poetry Rhyming Wheel is a tool you can use to help you find rhyming words.
RhymeZone.com and Rhymer are also useful Rhyming websites.