The rule of thumb when it comes to Shakespeare's plays is that the nobility (like Duke Senior) tend to speak in verse (poetry), which is a pretty formal way to talk. The commoners or, "Everyday Joes" (like Audrey), tend to speak just like we do, in regular old prose.
As You Like It breaks some rules. Rosalind (who is obviously a noble) tends to speak a lot of prose, especially when she's talking about love. In fact, over half of As You Like It is written in prose and the rest is written in iambic pentameter verse.
Here are some definitions and specific examples of prose and verse in As You Like It.
Like we said, the noble characters mostly speak in unrhymed iambic pentameter (also called "blank verse"). Don't let the fancy names intimidate you – it's really pretty simple once you get the hang of it. Let's start with a definition of iambic pentameter:
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.
Check out the play's opening lines, where Orlando admits that Rosalind has made him tongue-tied:
what PAssion HANGS these WEIGHTS upON my TONGUE? (1.2.9)
Every second syllable is accented (stressed) so this is classic iambic pentameter.
Like we said, ordinary folks don't talk in a special rhythm – they just talk. (This is especially true of country bumpkin types like Audrey.) Plus, in this play, some noble characters (like Rosalind and Celia) often speak both prose and verse. Here's an example of prose, where Rosalind and Celia talk privately about dreamy Orlando:
Celia: Why cousin, why Rosalind--Cupid have mercy, not a word?
Rosalind: Not one to throw at a dog. (1.3.1-2)
Why doesn't Rosalind speak in verse when she chats about Orlando? Probably because our girl Ros is very sensible and wants to keep artifice, formality, and BS to a minimum when she's having a little girl-talk with Celia. Still, that doesn't mean Rosalind can't speak in verse also. When Duke Frederick interrupts Ros and Celia's girl-talk, the two switch from prose to verse, which is a more formal and respectful way for them to talk to the Duke, who is also Celia's dad. (That's kind of like the way you have a different way of talking to friends than parents and teachers.)