I found this little paragraph that explains better than I can...
"DSL uses copper twisted pair wires just as regular telephone lines and ADSL typically shares the same wires voice communication uses. Normal telephone service often referred to as POTS (Plain old telephone service) uses the frequency range between 0 and 3,400 Hertz. Copper lines used for POSTS can handle a significantly higher range of frequencies, which transfers to how much data is possible over the wires. DSL uses some of the unused frequencies not used by POTS to transfer data. Because DSL has a larger amount of frequencies available to it, DSL is able to send and receive more data than a standard modem can that is limited to the 0 to 3,400 Hz frequency range."
Or check out this...
http://computer.howstuffworks.com/dsl.htm