Reading any one of Shakespeare's plays can feel like reading a really lengthy poem and that's because they're written in a combination of verse (poetry) and prose (how we talk every day). We break all of this down in the paragraphs that follow but here's what you should remember about Shakespeare's plays, Two Gentlemen included: the nobility tend to speak in verse, which is a pretty formal way to talk. The commoners, or "Everyday Joes" tend to speak just like we do, in regular prose. (Note: The play Richard II is the one exception to this rule – it's the only Shakespeare play written entirely in verse – even the gardeners speak poetry.)
In Two Gentlemen of Verona, the noble characters typically speak in iambic pentameter. (However, since this is likely Shakespeare's very first play, a lot of the meter tends to be pretty irregular.) Don't let the fancy name intimidate you – it's really pretty simple once you get the hang of it.
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:
ba-DUM, ba-DUM, ba-DUM, ba-DUM, ba-DUM
Let's try it out on this line, where Julia screams at her serving woman:
what FOOL is SHE that KNOWS i AM a MAID
Every second syllable is accented (stressed) so this is classic iambic pentameter.
The servants, like Speed and Lance, speak in prose, just like we talk every day. Prose is less formal than verse, so it's befitting of the servants' social status. Here's an example, where Lance tells the audience about how his dog peed under the Duke's table:
He thrusts me himself into the company of three or four
gentlemanlike dogs under the duke's table: he had
not been there--bless the mark!--a pissing while, but
all the chamber smelt him. 'Out with the dog!' says
one: 'What cur is that?' says another: 'Whip him
out' says the third: 'Hang him up' says the Duke. (4.4.1)
Lance's speech isn't fancy but it sure is entertaining.