The rhythm of free verse is like the rhythm of speech; it cares more for the sound of words and phrases than for particular repeated combinations of stressed and unstressed syllables, as in traditional poetry. Free verse also doesn't concern itself with rhymes.
The term free verse is a literal translation of the French term vers libre. The form was an innovation of late 19th-century French poets; American poets followed in their footsteps soon after. Most of the familiar 20th century poetry written in English is in free verse.
Free verse should not be confused with blank verse. We'll explain why next.