The style of The Canterbury Tales is characterized by rhyming couplets. That means that every two lines rhyme with each other. It's also in iambic pentameter (the same style as Shakespeare), meaning that in each line there are ten syllables, and a heavily emphasized (stressed) syllable follows a less emphasized (unstressed) syllable: [dah DAH] [dah DAH] [da DAH] [da DAH] [da DAH]. Each [da DAH] is an iamb, and there are five of them per line.
Chaucer's poetic style can be a little bit difficult because, a lot of the time, he twists his sentences around. As English-speakers, we're used to hearing the subject come first in the sentence, followed by the verb. But Chaucer will often do the opposite. Take the line "Than longen folk to goon on pilgrimages" (General Prologue 12). The subject, "folk," comes after the verb, "longen." Chaucer does this a lot, meaning that sometimes you have to wait until you get to the end of a line before you can really understand what's happening in the sentence. The reason for it is to help him keep his couplets rhyming, but darn does it make the Tales hard to read sometimes!