Here is a diagram of the internals of the MAX232A. It shows a double charge pump voltage doubler and a +10v to -10v voltage inverter. The voltages output are used to generate the RS-232 compliant signals. The MAX232A has provisions for two serial ports on the same physical package. Most people only connect one of them. You can get a data sheet for the MAX232 and friends from Maxim.
There are two camps on how to implement serial ports on a robot. One says stick the MAX232 on the robot. The other says don't. I kind of like the don't category! Most of my robots are autonomous, so I don't really need the MAX232 onboard consuming power.