Is The Canterbury Tales written completely in verse?
No, not all 20 stories from "The Canterbury Tales" are completely written in verse. As example, in fragment VII, "Sir Thopas" tales are written in prose.
As a modern poet, Chaucer has used the most common meter in English verse, called iambic pentameter. The line of iambic pentameter has 10 syllables and the accent falls at every second syllable. As the pentameter name indicates, there exists five feet in iambic pentameter, each foot consisting in a group of syllables. Similarly, William Shakespeare also used this meter of English verse.
Chaucer has used the couplet rhyme scheme in his tales and since they were written in iambic pentameter, they were called heroic couplets. But the couplet rhyme scheme is not present in all the tales, also quatrain or ballad stanza is other form that Chaucer has used in his tales.