Blank Verse (Unrhymed Iambic Pentameter)
In most of Shakespeare's plays, upper-class characters tend to speak in what's called "blank verse" or "unrhymed iambic pentameter," which is a pretty formal way to talk. (Don't freak about the fancy names because it's all actually pretty simple. You'll see why in a minute.) Commoners, or everyday Joes, tend to speak like we do, in regular old prose.
Since King John is all about royal families and their upper-crust political allies, it makes sense that most of the play is written in blank verse / unrhymed iambic pentameter, right?
So, let's break it down. An "iamb" is an unaccented syllable followed by an accented one. "Penta" means "five," and "meter" refers to a regular rhythmic pattern. So "iambic pentameter" is a kind of rhythmic pattern that consist of five iambs per line. It's the most common rhythm in English poetry and sounds like five heartbeats:
da DUM / da DUM / da DUM / da DUM / da DUM
Here's a famous example from the play, where King John gets all cold-blooded and orders Arthur's death.
HUBERT
My lord?
KING JOHN
A grave.
HUBERT
He shall not live.
KING JOHN
Enough.
Basically, Shakespeare takes one single line of blank verse and breaks it up between these two characters. Pretty cool (and creepy), don't you think? Here's how it looks when we put it together:
my LORD / a GRAVE / he SHALL / not LIVE / eNOUGH
Every second syllable is accented (stressed), so this is classic iambic pentameter. Since the lines have no regular rhyme scheme, we call it unrhymed iambic pentameter, a.k.a. blank verse.