Blank Verse and Prose
Here's what you should remember about Shakespeare's plays: The nobility tend to speak in "blank verse," which is essentially unrhymed poetry. The commoners tend to speak just like we do, in regular old prose.
OK. Now, let's think about Macbeth specifically.
Blank Verse or, Unrhymed Iambic Pentameter (The Nobles)
In Macbeth the noble characters mostly speak in unrhymed iambic pentameter, which is a fancy way of saying they talk like this:
ba-DUM, ba-DUM, ba-DUM, ba-DUM, ba-DUM.
See, 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. Let's try it out on this line, where Lady Macbeth urges her husband to wash his hands after he has murdered King Duncan:
and WASH this FILthy WITness FROM your HAND.
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.
Trochaic Tetrameter with Rhymed Couplets (The Witches)
The witches also speak in verse but it's done in a way that sets them apart from other characters. In fact, they often chant in a sing-song way that sounds a lot like a scary nursery rhyme. Many of their lines are delivered in what's called trochaic tetrameter with rhymed couplets.
That's a mouthful but, again, it's actually pretty simple once you wrap your brain around it. Let's take a closer look at "trochaic tetrameter."
A "trochee" is the opposite of an "iamb." It's an accented syllable followed by an unaccented syllable that sounds like DUM-da. "Tetra" means "four" and "meter" refers to a regular rhythmic pattern. So "trochaic tetrameter" is a kind of rhythmic pattern that consist of four trochees per line. It sounds like this:
DUM-da, DUM-da, DUM-da, DUM-da.
Here's an example from Macbeth:
DOUble, DOUble, TOIL and TROUble.
FIre BURN and CAULdron BUbble.
Notice the way the endings of these two lines rhyme (trouble and bubble)? That's what's called a rhymed couplet. On the one hand, the meter and the rhyme kind of make the chanting seem a little silly, especially for modern audiences, who don't necessarily believe in witchcraft. At the same time, all the talk about "hell-broth" and "trouble" sounds frightening, especially when what goes into the "hell-broth" consists of disturbing things like "eye of newt" and "finger of birth-strangled babe."
Prose (Commoners)
And now for the ordinary folk, like this poor hungover porter:
'Faith sir, we were carousing till the second
cock: and drink, sir, is a great provoker of three
things.
[…]
Marry, sir, nose-painting, sleep, and
urine. Lechery, sir, it provokes, and unprovokes; It provokes
the desire, but it takes away the performance. (2.3.24-26; 29-31)
Notice that it's not just the type of speech that sets the Porter apart from the nobles —it's also the content of what he says (which is "low" or "common"). Here, the Porter explains that he was up late "carousing" (partying) and then goes on to describe the physical consequences of excessive drinking: a red nose, a frequent urge to urinate, sleepiness, sexual desire, and problems "performing" in bed. Witty, sure—but it's not exactly what you'd call classy.