Does it have something to do with the shape of the frame? Was it just a factory code? I had a Chevy Cavalier once, and I was told it was a "J" body, and I hear mopar guys talk about "E" body, etc.
I am just curious as to how the carmakers come up with these letter designations. I tried to search the forums first, but of course the search engine can't understand that I am looking for threads referring to "A" body, and not "a body", haha.

Its a body designation. Thats it. They had to call them something, and assuming the "A" bodies were probably intended to be their volume sales models, They picked the letter "A".

Buick Skylark/GS, Chevrolet Chevelle, Olds Cutlass/442, Pontiac LeMans/Tempest/GTO all "A" bodies.

Just a factory code. It was an internal GM designation, uses for a particular chassis design... I doubt it was ever meant to be used outside GM (which is why Mopar A-bodies/B-bodies and E bodies shouldn't be confused)... You still here about this, but nowdays they use names like Zeta, sigma, etc and talk "architecture"...

Off the top of my head:
A-bodies: 1964-1977 Chevelles/Lemans/GTO (64-73)/Cutlass/Skylark
F-bodies: Camaros/Firebirds
X-Bodies: Novas and derivatives (70's Omegas and Venturas, 1974 GTO, etc)
B-bodies: full size
Y-bodies: Corvettes
E-bodies: Toronados, etc
G-bodies: 1978 and up Malibus, Monte Carlos/Regals/GNs, etc

Every auto manufacterer has its own line of body designation coding going back to the 30's (aka the old Model "A"). Todays codes are 2 or 3 digit and the imports tend to use alphanumeric codes.

Mopars body styles can get very confusing since they did alot of "cross badging" of cars in the 70's. There is a whole list of what's what for them going back to 1958 on bigblockdart.com. Alot of English cars carry 3-5 digit body styles for example, the Jaguar XKE body ran from 1961 I think to 1976, before that was the XK120, XK140, etc.

Here's a link to GM's future and current platform names:

http://www.gminsidenews.com/index.php?page=platform

the 78-81 malibus and what not were still callled "A" bodies, then the Citation got that designation, since it was supposedly the Malibu replacement.
i believe the Monte Carlo from 70 up to 88 was always a "G" body.
and what about the mighty "H" body? why does everyone always neglect the Vega and the Monza- which was supposed to be the all new for 1975 "F" body?