A blank verse poem is one which does not rhyme. However, it does have iambic pentameter, which refers to a rhythm where you have an unstressed syllable immediately followed by a stressed syllable. So, when you consider blank verse, think of it more as a 'beat', like a heart beat. For example, a heart beat rhythm goes something like this: Ba BOOM ba BOOM ba BOOM ba BOOM ba BOOM. Here, you can see that there exists an unstressed syllable, 'ba' followed by a stressed syllable, 'BOOM'. In the study of literature, this pattern of 'ba BOOM' is referred to as an iambic foot. Pentameter tells us that there are five, 'pentameter' feet to a line or as in our example five 'ba BOOM's per line. It is estimated that up to three-quarters of the world's poetry is written in blank verse.
While iambic pentameter is the most common form of blank verse, there are other types as well. These types include Trochee Blank Verse, Anapest Blank Verse, and Dactyl Blank Verse. In Trochee Blank Verse, you have a stressed/unstressed syllable rhythm. Anapest Blank Verse's rhythm is composed of unstressed/unstressed/stressed syllables, and Dactyl Blank Verse has stressed/unstressed/unstressed syllables.
If you've ever read John Milton's Paradise Lost, that is an example of a poem done in blank verse. This literary device is also used for monologues. This is where one character speaks his or her thoughts to an audience. One of the most famous monologues in history is from Shakespeare's Julius Caesar, which begins: Friends, Romans, countrymen, lend me your ears. So while blank verse usually uses iambic pentameter, it can be written in any meter, and have no exact number of lines. However, it is important to not confuse blank verse poetry with free verse poetry, as free verse follows no specific literary rules, having no meter or no set rhythm. Free verse is meant to represent freedom from literary rules, closely mirroring our natural speech patterns.