The difference between a DSL and a cable modem is that a DSL modem connects a computer to a phone line to transmit data, while a cable modem transmits data over existing TV lines. The further the DSL modem is from the ISP, the slower the connection; for cable modems, however, distance doesn't affect speed.
A DSL modem connects a computer to a single dedicated phone line to transmit digital data. If a computer has a voice band modem or internal modem, it can dial-in through the phone line. Otherwise, users must purchase an external modem to connect. DSL is faster than dial-up and allows users to access both the Internet and the phone line at the same time. DSL also allows users to choose the appropriate connection speed and price from among different providers.
A cable modem transmits data over coaxial cables that carry TV signals and can either be internal or external. A cable modem is usually faster than both dial-up and DSL.
Because a DSL modem transmits data directly from the ISP to the user, there is no bandwidth-sharing, meaning the transmission quality is fairly consistent. However, a cable modem delivers bandwidth in blocks to entire neighborhoods and shares it among homes, meaning speed drastically lowers during peak hours.