U can set up Your computer for fax services over internet or you can connect a normal fax-modem or fax-machine in the second pots port for normal operation as before. The former is free normal operation cost Telscum rates as well
I use my old fax/modem to send and receive faxes using Windows's in-built fax software (on my ADSL line). I also bought an el cheapo Canon scanner (R450) for scanning in the odd document that hasn't been electronically generated.
Since last week, I subscribed to Telkom's fax service (R10p/m) that routes incoming faxes to your email inbox - senders just have to prefix your normal land-line with 088.
ADSL uses the same line as the one you make/receive calls/faxes on. Nothing changes on the line except for the fact that you have a highly overpriced internet connection on it as well.
For ADSL, the phone line frequencies are split into a voice portion and a data portion. It is split at your house using microfilters. The voice portion can then carry anything it used to do, like phone calls, faxes, alarm system and even 56Kb modem connections. The rest of the frequencies on the line is then used for the ADSL conenction.
So, I would say there are very few people who do not use their ADSL line as their normal home/business phone line as well.