The USB is something totally different.
As for the serial pins...The RS-232 spec defines a number of things including the timing, but especially for our purposes the voltage. The serial pins on the Arduino use 0V for a logic 0 and 5V for a logic 1. RS-232 uses plus and minus voltages (minus is logic 1), and voltages near 0 are undefined. So you can do RS-232 with the Arduino using a simple voltage level converter (look at the MAX232 chip). RS-232 does not define what the bits mean (if you send a 65, that's an "A" in ascii, but it doesn't have to mean "A" with RS-232. And framing is not part of the spec, you don't need 8 bits in a byte.