If a poem has no regular rhyme scheme, it is called free verse. While many people think poems have to rhyme, this is not the case.
The term free verse refers not only to poetry that does not rhyme, but also to poetry that does not have a regular meter or rhythm to the words. As with all poetry, there are exceptions to the rule. Some free verse poetry does rhyme, such as the aptly titled "Free Verse" by Robert Graves. Conversely, there is also poetry that does not rhyme but is written in a rigid form, including the writings of Shakespeare. The poems were written in strict iambic pentameter or haiku, which are limited to a set amount of syllables.