These spoonfuls of alphabet-number soup designate different standards for connecting serial devices (like modems, mice, and printers) to the computer by plugging their cables into serial ports. Through a serial port, the computer exchanges information with the device back and forth "serially," or one bit at a time.
The RS stands for recommended standard as defined by the ERA (Electronics Industry Association). People mostly use names like RS-232 and Rs-422 to identify the type of serial ports on a computer, or the cables that connect them. "Ah, you've got an RS-232 port-you need an RS-232 cable." But technically, each standard defines a particular electronic system for passing information through a serial port-what voltages to use, that sort of thing not just the physical connections between your computer and a cable.
RS-232 is the most common type of serial port, the kind used on modems and the vast majority of pcs. It's simple and cheap, but unreliable at distances over 50 feet. The C in RS-232-C means it's the third version of the RS-232. This version is the most common. Rs-422 permits faster communications over longer distances. Rs-422 and 423 are both subsets of RS-499, which specifies which pins in the cable connector are supposed to do what.
But actually, these so-called standards aren't all that standardized. For example, the Macintosh serial port can serve as either an Rs-422 (for AppleTalk networks) or an RS-232 (for modems, and for serial, non- AppleTalk printers). However, the Mac uses incomplete versions of both standards, so you can't necessarily hook it up to a true-blue Rs-422 or RS- 232 serial device. This has become a particular problem with some of the newer high-speed modems. Anyway, you can look in a technical book if you want the details. In fact, you can ignore this RS business altogether when you buy a serial device, just make sure it's designed to work with your type of computer.