The tendency is to sugar prime as you may recall.
Hmmm, righty loosey lefty tighty... that's not what I learned in the service... thanks.
More questions ahead...
These responses are all very helpful -- for example, re euge's comment about sugar priming, I didn't pick that up at all from what I've been reading, not that my reading has been at all indepth or methodical. I just checked How to Brew, and as far as I can tell there's no mention of kegging (though the book is poorly indexed). I tend to discount Joy of Home Brewing as being fun but dated, but to my surprise it has an appendix on kegging I ignored back when I was focused on the basics, and the appendix discusses sugar-priming and other methods of carbonation.
The Toobs are highly variable; a two-page PDF from Morebeer never once mentions sugar-priming, though it has some helpful explanations of keg components I haven't seen elsewhere. The Homebrewopedia has this much to say about kegging: "Kegging.[keh'-ging] Drawing beer from a fermenter to kegs." A lot of homebrewing store websites appear to assume people know what they're buying.
I am also just figuring out that all that discussion about "ball lock" and "pin lock" refers to two different types of gas-in and beer-out connection points, if I'm right (I do get that used Cornie kegs generally have one or the other style, and that new kegs appear to be all ball lock). The lid of the keg appears roughly equivalent to a large bottlecap that isn't ever removed. The picnic tap is like a garden hose sprayer.
So there are several ways to carbonate beer in a keg. Ok. Now I'm trying to work out in my head the... physics??? of what's going on during the dispensing of a glass of kegged beer. Is it displacing the volume of beer dispensed from the keg with CO2 so that the keg remains full and the beer is always blanketed with a non-oxidizing gas? Does the CO2 help push out the beer from the keg, essentially displacing space in the keg so the beer has to come out of the keg via the line?
Re the total weight and my fridge capacity... I brew small batches because that's what I can easily lift and move, and I'm the only one in this home who likes beer. Even as separate components, I don't have the space or desire to be juggling 60 lbs + 20 lbs. Several homebrew stores sell 3-gallon or 2.5-gallon kegs, and some homebrew stores sell alternatives (I don't know how well they work) to the large CO2 containers. Through a miracle, we rent a place with a humongous, fairly new Kitchenaid fridge. I cook a lot and the two of us still don't fill that fridge, which means we're wasting energy chilling space. Having a small keg in the fridge at all times would be environmentally responsible! ;-)