I would be very wary of any crass generalization about personality types. I would be even warier of any statement that starts with, "There are two types of people..."
That having been said, there are two types of people -- "Windows People" and "A/C People." Don't over-literalize the origins, as it's a metaphor.
Here's where it comes from -- it's a hot (but not unbearably hot) day. Maybe it's the late spring or early summer. It's about 80 degrees with a mostly blue sky and maybe a slight breeze. You're driving around town or hanging out at your place. You could be thinking: a) what a beautiful day -- I'm going to take advantage of it by rolling down the windows and basking in this nice, natural air; or b) I better hermetically seal this car/house/apartment and crank the A/C on full blast before I (or a loved one) runs the risk of breaking a sweat -- yuck, how gross!
From that come the larger themes -- Windows People don't over-plan run-of-the-mill social engagements. They take pleasure in simple things. They're unafraid of public restrooms, door handles, and getting sand from the beach in their car. They don't count calories, they've never had a prescription for something that didn't end in -illin or -cycline, and they don't really give a rip about Britney Lohan-Hilton's boobjobchildimplantdivorcecourt proceedings. They're glad to have good-tasting food and they're not necessarily sure they'd know how to care about the way it's presented at a restaurant.
A/C People, on the other hand, are germaphobic, claustrophobic, and agoraphobic. They can't handle impromptu plan-making and they just plain don't like it when others "touch their stuff." They'll count the tip out to the last decimal place, and won't buy a round for the group until they're reminded to. They'll carry an argument out over years, let alone the appropriate number of seconds or minutes, even after you've conceded that yes, they were right. They were the kid with fifteen perfectly sharp number two pencils sitting next to you who snickered as you got downgraded to an "I" on the 5th grade science quiz for showing up without the proper writing instrument.
Obviously, you can see my bias towards Windows People in the way I've written this. Like I said, it shouldn't be over-literalized (hey, there are times when A/C sure is great) but it's my attempt at making a larger metaphor about the mentality with which people can go through life.