There are two kinds of characters in this play: 1) the kind who have sex outside of marriage and 2) the kind who don't. Those who do are pegged as outsiders, while those who don't are held up on a pedestal. Let's discuss.
Falstaff is a dude who runs around trying to seduce married women so he can swindle them out of their husbands' money. Obviously, he's not winning any popularity contests in Windsor. Our "merry wives" have lots of opportunities to cheat on their husbands with Falstaff but they don't, which is why Shakespeare is always telling us how "honest" they are.
Mistress Quickly, on the other hand, is a swingin' single. We're told that she's sleeping with her boss (1.2.3) despite the fact that she's also trying to help him romance a young, unmarried woman, Anne Page. That's why the "honest" housewives refer to her as "that foolish carrion" (3.3.163). "Carrion" meaning rotten flesh—the kind associated with nasty venereal diseases. Anne Page, on the other hand, is the poster girl for "pretty virginity" (1.1.39). Meaning? Every guy in the play wants to marry her.
In this play, middle-class characters (like the Fords and the Pages) tend to be associated with moral values. Here's the clearest example: Mistress Page and Mistress Ford are fun-loving and "merry" but they're also "honest" (aka faithful to their husbands). Their husbands are upstanding citizens who don't break the law and work hard to support their families financially. (We don't know what they do exactly but Shakespeare makes sure that we know they work for a living.)
Upper-class characters (like Falstaff and even Fenton) don't. They inherited their wealth or won it through fighting, and have blown it all before the play even begins. In other words? A serious lack of moral values. Falstaff's main goal in life is to get it on with rich, married women so he doesn't have to work. We're even told that Fenton was a wild child back in the day and has wasted all his family money doing who knows what.
Okay, so what about the servants? Well, they're not much better than the aristocrats. Most of them lie, cheat, and steal, and they don't care who knows about it. (Bardolph, Pistol, and Nim, we're talking about you here.) Go to "Themes: Society and Class" for more on this.
Sometimes Shakespeare just can't resist using really obvious names to help get his point across in this play. Pistol, for example, has a big mouth and an explosive personality, just like his name suggests. Mistress Quickly's name tells us that she's the kind of character who has lots of brief, sexual encounters with men. (Shakespeare loves to give shady ladies names like this—like Doll Tearsheet or Mistress Overdone.)
As for Master Shallow? Well, he's just that—shallow-minded, especially when he says his nephew should marry Anne Page for her money. Same goes for Master Slender—a guy who's got a skinny body, a small brain, and a "slender" vocabulary.
Don't even get us started on Simple. We're pretty sure you've got that covered.
Everybody in this play speaks prose (how ordinary people speak every day) but some do it better than others. Falstaff and the "merry wives" show us just how clever they are with their snappy word play and witty punning.
One guy speaks mostly verse (aka poetry) in this play and that's Fenton—the aristocrat who wants to marry Anne. Speaking verse is a pretty formal way to talk, so it fits with Fenton's social status and also his romantic pursuit of Anne.
Mistress Quickly is the polar opposite of Fenton. She speaks prose and is the queen of malapropism (the substitution of an incorrect word for one with a similar sound). At one point, she accidentally uses the word "erection" when she means to say "direction." In other words? She's a little bawdy and not too bright.
And let's not forget the foreigners. Evans and Caius tend to mispronounce words ("third" becomes "turd," etc.) and they often don't understand English slang, making it easy for other characters to tease them without them knowing it. But who are we really laughing at—Caius and Evans? Or the small-town xenophobes?