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        \     HTML by:        I The Winmodems Mini-tutorial I   Written by:     /

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       /      Martin L.       I_____________________________I   R a v e n       \

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First of all, what is a MODEM anyway? The initials modem stand for modulator/demodulator. To make a long story short, a modem translates digital signals which your computer sends in into signals that can be transferred through the phone line (if it's a cable modem or some other kind of modem, it'll translate computer signals into whatever signals that can be transferred through the communications line it is supposed to run on), and then receives back phone signals and translates them into bits and bytes your computer can read and understand.
Note: of course, your modem does more than that. It also provides the actual connection with the phone line.
This whole modulating (turning computer signals into phone signals) and demodulating (duh... the opposite of modulating. Turning phone signals into computer signals) process requires some software, and of course, a computer to run it. So your modem actually contains a little chip that does the modulating and demodulating.
Then what is a Winmodem? A Winmodem is a modem that does not do the modulating and demodulating thing. Instead, it leaves it up to your computer to do that (this is called software flow control, instead of hardware flow control in normal modems). It comes with the software necessary for your computer to perform these convertions.
So why do they suck so much? Because:
a) This whole convertion process takes up some resources from your computer, such as memory and CPU time.
b) If your computer slows down for some reason, you might get disconnected all of a sudden.
c) They don't run on any version of Unix. Why? Simply because there is no Winmodem software for Unix. The vendor didn't bother making them, I assume.
So why would a person want to buy a Winmodem? Simply because they cost less (I guess you could figure out why at this point).

Later addition: You can now go online from Linux with a WinModem! Check out http://www.linuxdoc.org/HOWTO/Winmodems-and-Linux-HOWTO.html for more information! Thanks to Striker-Man for pointing that out.